National Academies Press: OpenBook

Track Design Handbook for Light Rail Transit, Second Edition (2012)

Chapter: Chapter 4 - Track Structure Design

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Page 140
Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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Suggested Citation:"Chapter 4 - Track Structure Design." National Academies of Sciences, Engineering, and Medicine. 2012. Track Design Handbook for Light Rail Transit, Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22800.
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4-i Chapter 4—Track Structure Design Table of Contents 4.1 INTRODUCTION 4-1  4.2 TRACK AND WHEEL GAUGES AND FLANGEWAYS 4-1  4.2.1 Vehicle Truck Factors 4-1  4.2.2 Standard Track and Wheel Gauges 4-2  4.2.2.1 Railroad Gauge Practice 4-2  4.2.2.2 Transit Gauge Practice 4-3  4.2.2.3 Gauge Measurement Location 4-5  4.2.2.4 Gauge Issues—Joint LRT and Railroad and Mixed Fleets 4-6  4.2.2.5 Gauge Issues for Embedded Track 4-8  4.2.2.6 Non-Standard Track Gauges 4-9  4.2.3 Track Gauge Variation—General Discussion 4-9  4.2.4 Curved Track Gauge Analysis 4-11  4.2.4.1 Filkins-Wharton Flangeway Analysis 4-11  4.2.4.2 Nytram Plots—Truck-Axle-Wheel Positioning on Curved Track 4-14  4.2.4.2.1 Nytram Plot—Wheel Profile Sections 4-15  4.2.4.2.2 Nytram Plots—Static Condition 4-17  4.2.4.2.3 Nytram Plots—Dynamic Condition 4-18  4.2.4.2.4 Nytram Plots Considering Restraining Rail 4-20  4.2.5 Rail Cant and Wheel Taper—Implications for Track Gauge 4-23  4.2.5.1 Tapered Wheel Tread Rationale 4-24  4.2.5.2 Rail Grinding 4-26  4.2.5.3 Asymmetrical Rail Grinding 4-27  4.2.5.4 Variation of Rail Cant as a Tool for Enhancing Truck Steering 4-27  4.2.6 Construction and Maintenance Tolerances—Implications for Track Gauge 4-30  4.2.6.1 Tolerances—General Discussion 4-30  4.2.6.2 Tolerances and Track Gauge 4-31  4.2.6.3 Suggested Track Construction Tolerances 4-31  4.3 GUARDED CURVES AND RESTRAINING RAILS 4-32  4.3.1 Functional Description 4-33  4.3.2 Theory 4-33  4.3.3 Application Criteria 4-35  4.3.3.1 Non-Quantifiable Considerations for Restraining Rail 4-35  4.3.3.2 Longitudinal Limits for Restraining Rail Installations 4-37  4.3.4 Curve Double Guarding 4-38  4.3.5 Restraining Rail Design 4-38  4.3.5.1 Restraining Rail Working Face Angle 4-39  4.3.5.2 Restraining Rail Height 4-39  4.3.5.3 ADAAG Considerations for Restraining Rail 4-40  4.3.6 Omitting Restraining Rails—Pros and Cons 4-40  4.4 TRACK SUPPORT MODULUS 4-42 

Track Design Handbook for Light Rail Transit, Second Edition 4-ii 4.4.1 Modulus of Elasticity 4-42  4.4.2 Track Stiffness and Modulus of Various Track Types 4-44  4.4.2.1 Ballasted Track 4-44  4.4.2.2 Direct Fixation Track 4-45  4.4.2.3 Embedded Track 4-47  4.4.3 Transition Zone Track Modulus 4-48  4.4.3.1 Interface between Track Types 4-49  4.4.3.2 Transition Zone Track Design Details 4-49  4.4.3.3 Transition Zone Conditions 4-51  4.4.3.3.1 Transition from Ballasted Track to Direct Fixation Track 4-51  4.4.3.3.2 Transition from Ballasted Track to Embedded Track 4-51  4.4.3.3.3 Design Recommendation 4-52  4.5 BALLASTED TRACK 4-53  4.5.1 Ballasted Track Defined 4-53  4.5.2 Ballasted Track Criteria 4-54  4.5.2.1 Ballasted Track Rail Section and Track Gauge 4-54  4.5.2.2 Ballasted Track with Restraining Rail 4-54  4.5.2.3 Ballasted Track Fastening 4-54  4.5.3 Ballasted Track Structure Types 4-54  4.5.3.1 Ballasted Track Resilience 4-55  4.5.3.2 Timber Cross Tie Ballasted Track 4-56  4.5.3.2.1 Timber Cross Tie Rail Fastenings 4-56  4.5.3.2.2 Timber Cross Ties 4-57  4.5.3.3 Concrete Cross Tie Ballasted Track 4-58  4.5.3.3.1 Concrete Cross Tie Rail Fastenings 4-58  4.5.3.3.2 Concrete Cross Ties 4-59  4.5.4 Cross Tie Spacing 4-59  4.5.4.1 Cross Tie Spacing—Vertical Support Considerations 4-59  4.5.4.2 Cross Tie Spacing—Lateral Stability Considerations 4-61  4.5.5 Special Trackwork Switch Ties 4-62  4.5.5.1 Timber Switch Ties 4-62  4.5.5.2 Concrete Switch Ties 4-63  4.5.6 Ballast and Subballast 4-64  4.5.6.1 Ballast Depth 4-64  4.5.6.2 Ballast Width 4-64  4.5.6.3 Subballast Depth and Width 4-65  4.5.6.4 Subgrade 4-66  4.5.7 Ballasted Track Drainage 4-66  4.5.8 Retained Ballasted Guideway 4-67  4.5.9 Stray Current Protection Requirements 4-67  4.5.10 Ballasted Special Trackwork 4-68  4.5.11 Noise and Vibration 4-68  4.5.12 Signal/Train Control System 4-68  4.5.13 Traction Power 4-69  4.5.14 Grade Crossings 4-69 

Track Structure Design 4-iii 4.6 DIRECT FIXATION TRACK (BALLASTLESS OPEN TRACK) 4-70  4.6.1 Direct Fixation Track Defined 4-70  4.6.2 Direct Fixation Track Criteria 4-71  4.6.2.1 Direct Fixation Track Rail Section and Track Gauge 4-71  4.6.2.2 Direct Fixation Track with Restraining Rail 4-71  4.6.2.3 Direct Fixation Track Rail Fasteners 4-71  4.6.2.4 Track Modulus 4-71  4.6.3 Direct Fixation Track Structure Types 4-71  4.6.3.1 Reinforced Concrete Plinths 4-73  4.6.3.1.1 Concrete Plinth in Tangent Track 4-74  4.6.3.1.2 Concrete Plinth in Superelevated Curved Track 4-75  4.6.3.1.3 Concrete Plinths with Restraining or Emergency Guard Rail 4-75  4.6.3.1.4 Concrete Plinth Lengths 4-77  4.6.3.1.5 Concrete Plinth Height 4-78  4.6.3.1.6 Plinths on Decks Twisted for Superelevation 4-79  4.6.3.1.7 Direct Fixation Vertical Tolerances 4-79  4.6.3.1.8 Concrete Plinth Reinforcing Bar Design 4-79  4.6.3.2 Cementitious Grout Pads 4-82  4.6.3.2.1 Cementitious Grout Pad on Concrete Surface 4-83  4.6.3.2.2 Cementitious Grout Pad in Concrete Recess 4-84  4.6.3.2.3 Cementitious Grout Material 4-84  4.6.3.3 Direct Fixation “Ballastless” Concrete Tie Block Track 4-85  4.6.3.4 Plinthless Direct Fixation Track 4-86  4.6.4 Direct Fixation Fastener Details at the Rail 4-87  4.6.5 Direct Fixation Track Drainage 4-88  4.6.6 Direct Fixation Stray Current Protection Requirements 4-89  4.6.7 Direct Fixation Special Trackwork 4-90  4.6.8 Noise and Vibration 4-90  4.6.9 Direct Fixation Track Communication and Signal Interfaces 4-90  4.6.10 Overhead Contact System—Traction Power 4-91  4.7 EMBEDDED TRACK DESIGN 4-91  4.7.1 Embedded Track Defined 4-92  4.7.2 Embedded Rail and Flangeway Criteria 4-93  4.7.2.1 Embedded Rail Details at the Rail Head 4-94  4.7.2.2 Wheel/Rail Embedment Interference 4-95  4.7.3 Embedded Track Types 4-96  4.7.3.1 Non-Resilient Embedded Track 4-97  4.7.3.2 Resilient Embedded Track 4-98  4.7.3.3 Floating Slab Embedded Track 4-99  4.7.3.4 Proprietary Resilient Embedded Rail Designs 4-100  4.7.4 Concrete Slab Track Structure 4-100  4.7.4.1 Embedded Rail Installation 4-102  4.7.4.1.1 Top-Down Construction—Rail Support and Gauge Restraint 4-102  4.7.4.1.2 Floating Rail Installation 4-105  4.7.4.1.3 Alignment Control in Top-Down Construction 4-105 

Track Design Handbook for Light Rail Transit, Second Edition 4-iv 4.7.4.1.4 Bottom-Up Embedded Rail Installation 4-106  4.7.4.2 Stray Current Protection Requirements 4-107  4.7.4.3 Rail Insulating Materials 4-109  4.7.4.3.1 Extruded Elastomeric Rail Boot and Trough Components 4-110  4.7.4.3.2 Resilient Polyurethane 4-111  4.7.4.3.3 Elastomer Pads for Rail Base 4-112  4.7.4.3.4 Elastomeric Fastenings (Direct Fixation Fasteners) 4-112  4.7.4.3.5 Concrete and Bituminous Asphalt Trough Fillers 4-112  4.7.4.4 Embedded Track Drainage 4-112  4.7.4.4.1 Surface Drainage 4-114  4.7.5 Ballasted Track Structure with Embedment 4-116  4.7.6 Embedded Special Trackwork 4-119  4.7.7 Noise and Vibration 4-121  4.7.8 Transit Signal Work 4-122  4.7.9 Traction Power 4-122  4.7.10 Turf Track 4-122  4.8 LRT TRACK ON BRIDGES 4-125  4.9 REFERENCES 4-125  List of Figures Figure 4.2.1 AAR-1B narrow flange wheel 4-3  Figure 4.2.2 Suggested standard wheel gauge—transit system 4-5  Figure 4.2.3 Gauge line locations on 115 RE rail head 4-6  Figure 4.2.4 Filkins-Wharton diagram for determining flangeway widths 4-13  Figure 4.2.5 Filkins-Wharton plot to establish flangeways 4-14  Figure 4.2.6 Wheel sections for Nytram plot—oblique view 4-15  Figure 4.2.7 Wheel sections for Nytram plot—modified AAR-1B transit wheel 4-16  Figure 4.2.8 Static Nytram plot 4-18  Figure 4.2.9 Nytram plot—rotated to first point of contact 4-19  Figure 4.2.10 Nytram plot—rotated to second point of contact 4-20  Figure 4.2.11 Static Nytram plot with restraining rail 4-21  Figure 4.2.12 Nytram plot with restraining rail—rotated to first point of contact 4-22  Figure 4.2.13 Nytram plot with restraining rail—rotated to second point of contact 4-22  Figure 4.2.14 Rail cant design and wheel contact 4-29  Figure 4.4.1 Track transition slab 4-50 Figure 4.5.1 Ballasted single track, tangent track (concrete cross ties) 4-56

Track Structure Design v-4 Figure 4.5.2 Ballasted single guarded curve track (concrete cross ties) 4-57 Figure 4.5.3 Ballasted double tangent track (concrete cross ties) 4-58 Figure 4.5.4 Ballasted double curved track (concrete cross ties) 4-59 Figure 4.5.5 Ballasted track—curbed section 76-4 Figure 4.6.1 Concrete plinth design—tangent direct fixation track 4-74 Figure 4.6.2 Concrete plinth design—graduated J-bars to match superelevated plinth heights 67-4 Figure 4.6.3 Concrete plinths—superelevated track with restraining rail 4-77 Figure 4.6.4 Concrete plinth lengths 77-4 Figure 4.6.5A Concrete plinth reinforcing bar details 08-4 Figure 4.6.5B Concrete plinth reinforcing bar details (continued) 4-81 Figure 4.6.6 Cementitious grout pad design—direct fixation track 4-83 Figure 4.6.7 Independent dual-block concrete tie track system 58-4 Figure 4.6.8 Rail cant and base of rail positioning 88-4 Figure 4.7.1 Embedded rail head details 59-4 Figure 4.7.2 Embedded track on leveling beams 101-4 Figure 4.7.3 Concrete slab with individual rail troughs 201-4 Figure 4.7.4 Floating rail embedment—base material installation 4-105 Figure 4.7.5 Rail fastening installations 701-4 Figure 4.7.6 Extruded elastomer trough and rail boot for tee rail 4-109 Figure 4.7.7 Polyurethane trough filler with web blocks 111-4 Figure 4.7.8 Typical embedded track drain chase 4-1 Figure 4.7.9 Depressed pavement without flangeways 611-4 Figure 4.7.10 Ballasted track structure with embedment 711-4 Figure 4.7.11 Bituminous pavers with sealed joints 811-4 Figure 4.7.12 Use of brick or stone pavers with embedded tee rail 4-118 Figure 4.7.13 Special trackwork—embedded “bathtub” design 4-120 Figure 4.7.14 Turf track 521-4 List of Tables 23-4 secnarelot noitcurtsnoc kcarT 1.2.4 elbaT 16-4 sretemarap ngised kcart detsallaB 1.5.4 elbaT 13

4-1 CHAPTER 4—TRACK STRUCTURE DESIGN 4.1 INTRODUCTION The design standards for contemporary light rail transit (LRT) track structures, whether in an at- grade, aerial, or tunnel environment, differ considerably from the principles for either “heavy” rail transit or railroad service. The varied guideway environments in which an LRT system can be constructed result in horizontal and vertical track geometry that often affects light rail vehicle (LRV) design and performance. Consequently, the light rail track designer must consider not only the track geometry, but also the design characteristics of the LRV and how it responds to the guideway geometry. This is particularly true in embedded track located in streets. In general, construction of an LRT guideway in a city street constitutes the greatest challenge to the light rail track designer. 4.2 TRACK AND WHEEL GAUGES AND FLANGEWAYS The determination of the correct dimensions to be used for track gauge and wheel gauge and for the widths of the flangeways through special trackwork and other guarded portions of the track structure is the most crucial activity to be undertaken during track design. If these design dimensions are not carefully selected to be compatible with the rail vehicle(s) that will operate over the track, unsatisfactory performance and excessive wear of both the track structure and the vehicle wheels will occur. 4.2.1 Vehicle Truck Factors New, state-of-the-art LRV designs, particularly “low-floor” LRVs, incorporate many features radically different from high-floor LRVs, heavy rail metros, and railroads. These can include smaller diameter wheels, short stub axles with independently rotating wheels (IRWs), and a wide variety of truck axle spacings and truck centers—all of which affect the vehicle’s interface with the track structure. In many cases, multiple variations of these factors can occur on a single articulated car. A common situation involves a shorter truck wheelbase on the center non- powered truck of a partial low-floor light rail vehicle. Smaller diameter wheels may also be introduced, and the trams in one European capital city even have two different wheel diameters on the same truck! If these parameters are not carefully considered in the track design, the vehicle’s operational tracking pattern can be susceptible to hunting, center truck severe skewing in curves, and unpredictable center truck action at special trackwork. The relationship of track gauge to wheel gauge, particularly the back-to-back (“B2B”) dimension between the wheels, is especially important in controlling these operational performance features. In general, reducing the lateral clearance between the wheel flange and rail head gauge face, either through increasing the wheel gauge (preferred) or decreasing the track gauge, improves wheel tracking of the rail in curves by keeping the truck/wheel as square to the rails as possible. This reduces hunting, skewing, and flange attack angle and results in improved performance through curved track and special trackwork. Vehicle wheel gauge will generally not vary within a given LRV fleet, although cases have occurred where the wheel gauge and wheel profile of a new vehicle procurement have not matched that of the transit agency’s existing fleet. It is extremely

Track Design Handbook for Light Rail Transit, Second Edition 4-2 important that the track designer take steps to ensure that the vehicle designer does not select wheel parameters independent of track design. If, as is common, there are several series of vehicles in use on a rail transit system, each with a different combination of truck characteristics, the track designers must consider the worst-case requirements of each car series and optimize the track gauge parameters accordingly. 4.2.2 Standard Track and Wheel Gauges The majority of contemporary rail transit systems nominally utilize “standard” track gauge of 56 ½ inches [1435 mm]. This track gauge stems from 18th-century horse drawn railways used by English collieries, where track gauge was dictated by the common wheel-to-wheel “gauge” of the wagons used to haul the coal. While many different track gauges were adopted over the years, none have proven to be either as popular or practical as standard gauge. Track that is nominally constructed to standard gauge can actually be tighter or wider than 56 ½ inches [1435 mm] depending on a variety of circumstances. The track gauge can be adjusted along the route so as to optimize vehicle-to-track interaction. Conditions that can require gauge adjustments include track curvature, the presence or lack of curve restraining rails, and several vehicle design factors. Vehicle factors include wheel diameter, wheel tread taper and width, wheel flange shape including both height and thickness, the distance between axles (also known as “wheelbase”), and the wheel gauge distance between wheels mounted on a common axle. While nominal 56 ½ inch [1435 mm] standard track gauge is nearly universal for both electric rail transit and “steam” railroads, the different requirements of these modes resulted in appreciably different details, such as where the track gauge is measured, under what conditions it is varied, and the amount of freeplay that is required between the wheel flanges and the gauge faces of the rails. 4.2.2.1 Railroad Gauge Practice North American railroads set track and wheel mounting gauges in accordance with criteria established by the Mechanical Division of the Association of American Railroads (AAR) and the American Railway Engineering and Maintenance-of-Way Association (AREMA). As shown on AREMA Plan basic number 793, AAR standard wheel gauge is defined as 55 11/16 inches [equivalent to 1,414 millimeters] and is measured 5/8 of an inch [15.9 millimeters] below the wheel tread surface. The AREMA definition of track gauge is measured at the same distance below the top of rail. These gauge standards have been incorporated into many contemporary LRT track designs to accommodate possible joint railroad and LRT operations. AAR promulgates two wheel profiles. The AAR-1B Narrow Flange wheel is designed for locomotives and passenger equipment. The AAR-1B Wide Flange wheel is intended only for freight cars. If wheels using the AAR-1B Narrow Flange wheel are mounted at standard AAR wheel gauge and the wheel and axle assembly is centered between the rails at standard track gauge, the horizontal clearance between the wheel and the rail at the gauge line elevation is 13/32 inch [10.3 mm] as shown in Figure 4.2.1. This results in total freeplay between correctly mounted (and unworn) wheelsets and exactly gauged rails of 13/16 inch [almost 21 millimeters].

Track Structure Design 4-3 For trucks with conventional solid axles and not independently rotating wheels, the freeplay assists in the steering or curving of the axle by the differential in wheel diameters, provided the wheel treads are tapered. See Article 4.2.4.1 for additional discussion on this point. Figure 4.2.1 AAR-1B narrow flange wheel (superimposed on 115 RE and 59R2 rails) It is important to recognize that railroad gauge practices generally evolved in a different environment than transit operations. Particularly for curved tracks, railroad criteria are predicated on the use of equipment that generally has much larger diameter wheels than those used on transit vehicles. In addition, both the maximum wheelbase and the number of axles that might be mounted on a rigid truck frame are usually much greater. Steam locomotives in particular could have wheels over 6 feet [1.8 meters] in diameter, with up to five such sets of wheels on a rigid frame. Even contemporary diesel locomotives can have wheels that are 42 inches [almost 1.1 meters] in diameter, with three wheel and axle sets on trucks that can have an overall wheelbase of 13 feet [nearly 4 meters]. By contrast, contemporary rail transit vehicles rarely have wheels over 28 inches [711 mm] in diameter, never have more than two axles per truck, and generally have maximum wheelbase distances no longer than 6.00 to 6.25 feet [1800 to 1900 mm]. Only one U.S. LRT system has a longer wheelbase, and it occurs on a unique vehicle design that is unlikely to ever be duplicated. The much larger truck features associated with railroad equipment dictate relationships between wheel gauge and track gauge that are far less constrained than those required for transit equipment. In addition, freight car wheel maintenance tolerances both for wheel contour and back-to-back (“B2B”) wheel gauge are far looser than those of insular transit systems. Freight track must therefore be more forgiving. Hence, it is recommended that railroad track gauge, wheel gauge, and flangeway width criteria not be adopted for an LRT track system unless both transit and freight railroad equipment will operate jointly on a common track. 4.2.2.2 Transit Gauge Practice Traditional street railway/tramway systems developed guidelines for wheel gauge that differ considerably from guidelines used by railroads. In the United States, the most common standards for track and wheel mounting gauges were those promulgated by the American Electric

Track Design Handbook for Light Rail Transit, Second Edition 4-4 Railway Engineering Association (later renamed the American Transit Engineering Association or ATEA). The ATEA standard track and wheel gauges were 56 ½ and 56 ¼ inches [1,435 and 1,428 millimeters], respectively, and were measured ¼ inch [6 millimeters] below the top of the rail. In addition, some transit systems tightened the track gauge in tangent track, taking advantage of a compound curve gauge corner radius that was rolled into the head of some ATEA girder rails. The few “legacy” North American light rail systems that predate the 1970s renaissance of light rail transit typically follow wheel gauge standards that can be traced back to ATEA recommendations. European tramways developed similar standards, although it is important to note that, in general, European street railways use wheel flanges that are even smaller than those promulgated by ATEA. The transit type standards for wheel gauge have several advantages: • With a tighter gauge relationship, truck hunting—the lateral oscillation of a truck from one rail to the other as it seeks a consistent rolling radius on all wheels—is more easily controlled. Hunting typically is a tangent track phenomenon and is more prevalent at higher vehicle speeds. Hunting has multiple causes, including the spring rate of the truck’s primary suspension. • Trucks cannot become as greatly skewed to the track, thereby reducing the angle of attack between the wheel flange and the gauge face of the rail (also known as “flange bite”) in tangent and curved track. • Flangeways can be appreciably narrower, a significant consideration for embedded track areas with significant pedestrian activity. This coincidently permits the use of groove rails with relatively narrow flangeways when desired. Generally, tight wheel-gauge-to-track-gauge relationships can only be employed when the transit operator does not have to share its tracks with a railroad. There are exceptions in Europe where the transit systems have implemented special designs of wheels and special trackwork to permit “tram-train” LRT operations. These systems use tramway tracks in city streets and switch to freight railroad tracks in suburban areas. The first such operation was in Karlsruhe, Germany, and several other transit systems have implemented similar services. Most North American LRT systems do not share track with freight railroads. Since they are thus not restricted by AAR practices, they feature a wide variety of vehicle wheel profiles and gauges even though most employ standard track gauge of 56 ½ inches [1435 millimeters]. As a guideline, Figure 4.2.2 illustrates a suggested wheel gauge for transit use with standard track gauge of 56 inches [1422.4 millimeters]. Use of this wheel gauge results in ½ inch of total freeplay, which is effectively a compromise—5/16 inch [8 mm] less than AAR wheel gauge practice but ¼ inch [6 mm] more than the freeplay endorsed by the former ATEA. The freeplay between each wheel and the rail it is riding on is therefore ¼ inch [6.35 millimeters]. Readers should compare Figure 4.2.2 against Figure 4.2.1 to see the differences between railroad and transit wheel gauge practice. In particular, note that the transit wheel illustrated in Figure 4.2.2 uses a thinner flange than the AAR- 1B wheel. Because of this difference, the B2B dimension on the transit wheelset is ⅞ inch [22 mm] larger than AAR practice. The combination of thinner flanges

Track Structure Design 4-5 and a larger B2B dimension is what permits LRT operations to successfully use groove rails with narrow flangeways. See Article 4.2.2.5 for additional discussion related to freeplay and the use of narrow flangeway groove rails. Figure 4.2.2 Suggested standard wheel gauge—transit system 4.2.2.3 Gauge Measurement Location Track gauge is measured a specific distance below top of rail because of the gauge corner radii of the rail and the flange-to-tread fillet radius of the wheel. The location where gauge is measured frequently differs between railroad and transit systems. The customary gauge elevation point on North American railroads is 5/8 inch [15.9 millimeters] below top of rail. Track gauge on traditional street railway systems was, and in some instances still is, measured at either ¼ inch [6.4 millimeters] or 3/8 inch [9.5 millimeters] below top of rail. Rail sections with compound gauge corner radii, such as 115 RE section (see Figure 4.2.3), do not have a nominally vertical tangent section for gauge measurement at the ¼-inch [6.4-mm] or 3/8-inch [9.5-millimeters] height, hence the designation of a lower elevation. Older rail sections that were prevalent when the ATEA promulgated its standards, such as ASCE, ARA-A and ARA-

Track Design Handbook for Light Rail Transit, Second Edition 4-6 B rail sections, had smaller gauge corner radii and thus were more conducive to gauge measurement closer to top of rail. Such rail is no longer commonly rolled in North America. Since measurement of gauge within the curved portion of the rail head is difficult at best and misleading at worst, it is recommended that gauge elevation be defined consistent with railroad practice. North American transit systems should therefore designate gauge elevation at 0.625 inches [15.9 millimeters] below top of rail for tee rail. Figure 4.2.3 Gauge line locations on 115 RE rail head European practice for gauge line elevation ranges from 10 to 15 millimeters [0.39 to 0.59 inches] depending on the source of the information. A gauge line elevation of 10 millimeters [about 3/8 inch] is inappropriate simply because it is still within the gauge corner radius of the rail head. Moreover, any differences between 15 mm and 5/8 inch would be totally masked by ordinary fabrication and construction tolerances. The researchers believe that for systems using modern rail sections with compound gauge corner radii, such as 115 RE tee rail and 60R2 groove rail, the North American convention of 5/8 inch [15.9 millimeters] is an appropriate elevation for measuring track gauge. 4.2.2.4 Gauge Issues—Joint LRT and Railroad and Mixed Fleets For a system with a mixed fleet, compromises may be required to accommodate a variety of truck and wheel parameters. This problem is not new—early 20th-century electric street railway track designers frequently had to adapt their systems to handle not only city streetcars with short wheelbase trucks and relatively small diameter wheels, but also “interurban” trolleys that typically had longer wheelbase trucks and larger diameter wheels. Some trolley companies even offered freight service and routinely handled “steam” railroad freight cars over portions of their lines. Today, if a light rail system shares any portion of its route with a freight railroad, or if future extensions either will or might share freight railroad tracks, then conformance with freight railroad gauge and other freight geometry constraints may control some elements of the track design. When a new light rail system shares track with a freight railroad, freight operations normally occur only along ballasted track segments. It is unusual for freight trains to share aerial structure or embedded track segments of a system. In general, the mixing of rail freight and LRT operations on any portion of a system will govern track and wheel gauge design decisions for the entire system unless Karlsruhe-type compromise wheels and special trackwork designs are adopted. Compromises will be required both on the vehicle and on the shared track and may have some effect on the transit-only portion of track on the same system as well. Even if the system’s “starter line” does not include joint operation areas, consideration should be given to whether future extensions of the system might share tracks with a freight railroad.

Track Structure Design 4-7 Regardless of whether or not joint operation with a freight railroad is contemplated, there are several key issues to consider. These include the setting of the back-to-back wheel dimension, guard check gauge, and guard face gauge criteria that result from a particular wheel setting. Track design parameters that will be most affected by these decisions include • The practicality of using available groove and guard rails that are rolled with a specific flangeway width. • The flangeway width and track gauge required for effective restraining rail or guard rail applications. • Details for guarding of frog points (both turnouts and crossing diamonds) in special trackwork locations. Transit systems that do not share tracks with a freight railroad may still have a track connection at the maintenance facility yard for delivery of freight cars loaded with track materials or the system’s new light rail vehicles. If the system’s maintenance plan contemplates movement of railroad rolling stock (such as hopper cars full of ballast) over portions of the system, it may be necessary to compromise the track design to accommodate the railroad equipment. This does not mean wholesale adoption of railroad standards. Provided that the guard check gauge at turnout frogs allows sufficient space for AAR back-to-back wheel gauge, freight cars can usually be moved over open track portions of an LRT system at low speeds. It may be necessary to prohibit any railroad equipment with wheels that are not precisely mounted, as maintenance tolerances for railroad wheel settings are considerably more liberal than those applied to rail transit fleets. AAR standard wheel profiles and wheel gauge on railroad equipment is a very important issue when considering occasional operation of railroad equipment over a track system designed for LRT-only service. Embedded track areas that utilize narrow flangeway groove rails typically cannot accommodate movements of railroad rolling stock through curves with radii less than about 300 feet [approximately 91 meters]. Groove rails with wide flangeways that can accommodate freight rolling stock are available, but the flangeways are wider than desirable. See Chapter 5 for additional information. Other restrictions on railroad equipment movements involve the structural capacity of bridges designed for LRT loads and clearances to trackside obstructions such as catenary poles and station platforms. Another category of joint operations is where it is proposed to extend an existing “heavy” rail transit operation using light rail technology. The existing system will already have track gauge, wheel gauge, and wheel profile standards in place that must be considered in the design of the light rail tracks and vehicles for the new system. If the truck parameters of the existing rolling stock, such as truck wheelbase or wheel diameter, are appreciably different from typical LRV designs, compromises will be necessary to achieve compatible operations. Special consideration must be given to existing maintenance-of-way vehicles, such as hy-rail trucks, since their wheel profiles and mounting dimension may be inconsistent with the new extension’s track design. Even if neither railroad rolling stock nor mixed transit car fleets are a consideration, the trackwork designer should consider the ramifications that track and wheel gauge variations might have for on-track maintenance-of-way equipment. It is imperative that specific notification be given that

Track Design Handbook for Light Rail Transit, Second Edition 4-8 the transit system’s gauge standards differ from AAR and AREMA standards so that construction and maintenance equipment do not damage the track. Refer to Chapters 13 and 14 for more on this subject. 4.2.2.5 Gauge Issues for Embedded Track The appropriate track gauge to use in embedded track is highly dependent on the rail section (either tee rail or groove rail) and the vehicle wheel gauge. In this regard, it is very important to note that standard railroad wheel contours (e.g., AAR-1B) and railroad wheel mounting gauges are not compatible with narrow flangeway groove rails presently available from European mills if the track is built to 56-½-inch [1435-millimeter] gauge. The backs of the wheels will bind with the tram or guarding lip of the groove rail causing one flange to ride up out of the flangeway. If narrow flangeway groove rails—such as 51R1, 53R1, 59R2, and 60R2—are selected, it will be necessary to adopt either a wide wheel gauge or an equivalent narrow track gauge. Narrowing the overall track gauge to something less than standard was occasionally employed on legacy rail transit systems, but is no longer a common practice. It could be considered under extenuating circumstances, but, in general, it is not recommended due to the impact on all equipment required to maintain the track system. Embedded track is typically separated from joint use track. However, if railroad standard wheel gauge must be employed on an LRV because some portion of the route shares track with a freight railroad, wheel clearance to the embedded groove rail track can alternatively be achieved by reducing the track gauge only in those areas where the groove rail is installed. This will reduce the wheel-rail clearance at the gauge line (“freeplay”), alter the rail/wheel interface compared to other portions of the route, and may result in unsatisfactory interaction with both transit and railroad equipment. It may be possible to mitigate these issues by adopting special rail-grinding profiles in any areas of tightened gauge; however, note that rail grinding in embedded track areas is more difficult in any event. Railroad equipment movements that are limited to occasional maintenance work trains at low speed may be acceptable. The above measures should only be considered after detailed study. Also, note that the track designer will have no control over the condition of the wheels of any freight equipment that operates over nominally LRT-only track. The track designer cannot safely assume that operations and maintenance personnel responsible for any such possible future movements will diligently scrutinize the condition of the wheels of any interchange equipment and reject those that do not comply with some standard higher than AAR’s interchange rules. If routine joint operation with railroad freight equipment along an embedded track area is expected, use of narrow flangeway groove rails will not be possible. Wide flangeway groove rails for freight railroad use are provided by some European rolling mills, but, presently, available designs of this type have flangeways that are so wide and tram height that is so low that they cannot provide any appreciable guarding action for curves or special trackwork. This was not the case with girder guard rails made in North America until the mid-1980s; however, these rails can no longer be obtained. A near match of the head and flangeway contours of the former North American designs can be achieved by milling the head of one of the structural groove rail sections available from European mills; however, this is an expensive solution that requires careful investigation and justification. See Chapter 5 for discussion of procurement issues related to European groove rails.

Track Structure Design 4-9 More latitude for joint operations in embedded track can be achieved using tee rails rather than groove rails; however, a separate flangeway must be constructed and maintained in the pavement surface. Refer to Chapter 5 of this Handbook for additional discussion concerning the application of tee rails to embedded track. 4.2.2.6 Non-Standard Track Gauges In addition to standard 56-½-inch [1,435-millimeters] track gauge, several other gauges have been used on light rail transit systems in North America and overseas. Narrow gauge systems, typically meter gauge [39.37 inches], are relatively common in Europe, particularly in older cities where narrow streets restrict vehicle sizes. There were once many narrow gauge street railways in North America; however, the only survivors are the San Francisco cable car system and a trolley museum near Los Angeles. Broad gauge trolley systems were more common, and, for a period back in the 1960s and 1970s, there were actually more miles of broad gauge streetcar track in North America than there were standard gauge trolley lines. Four legacy streetcar operations in North America use broad track gauges. These range from 58 7/8 inches [1,496 millimeters] in Toronto to 62 ¼ inches [1,581 millimeters] in Philadelphia and 62 ½ inches [1,588 millimeters] on the Pittsburgh and New Orleans systems. Such odd gauges were typically dictated by the municipal ordinances that granted the streetcar companies their “franchise” to operate within the city streets. In such legislation, it was typically specified that the rails should be laid at a distance apart that conformed to local wagon gauge, thereby providing horse drawn wagons and carriages with a smoother running surface than the primitive pavements of the era. The only new start transit operation in North America to adopt a non-standard track gauge in recent years was San Francisco’s BART “heavy” rail system at 66 inches [1,676 millimeters]. This gauge was reportedly intended to provide increased vehicle stability against crosswinds for a proposed but never built bridge crossing of San Francisco Bay. Those systems that employ unusual gauges typically rue the fact because it complicates many facets of track and vehicle design, construction/fabrication, and maintenance. Contracting for services such as track surfacing and rail grinding becomes more difficult and expensive since contractors do not routinely have broad gauge equipment on hand and converting and subsequently reverting standard gauge equipment for a short-term assignment is time consuming and expensive. Vehicle procurement is also complicated since off-the-shelf truck designs must be modified, and potential savings from joint vehicle procurements cannot be realized. Wide gauges also preclude joint operation of a rail transit line on a railroad route since dual gauge special trackwork and the train control systems necessary to operate it are both extremely complex and expensive. Accordingly, non-standard gauges are not recommended for new start projects. Systems which presently have broad gauge track most likely need to perpetuate that practice for future extensions so as to maintain internal compatibility in both track and rolling stock design. Notably, Toronto’s “Transit City” LRT expansion program is utilizing standard track gauge as it has no interface with their legacy streetcar system. 4.2.3 Track Gauge Variation—General Discussion Light rail transit tracks that are constructed with conventional tee rails and operate only light rail vehicles with conventional wheelbase trucks and wheel diameters can use standard 56-½-inch

Track Design Handbook for Light Rail Transit, Second Edition 4-10 [1,435-millimeter] track gauge in both tangent track and virtually all radius curves without regard to whether railroad or transit design standards are used for wheel gauge. On an ideal light rail system, there would be no need for any variations of the track gauge, thereby producing a completely uniform environment for the wheel/rail interface. This may not be practical, particularly on systems that have tight radius curves and/or employ narrow flangeway groove rails. When mixed track gauges are employed, the designer should consider rail-grinding operations and the adjustment capabilities of state-of-the-art rail-grinding machines as a means of maintaining a reasonably consistent wheel-rail interface pattern. The threshold radius at which it may be appropriate to alter the gauge in curved tracks will vary based on a number of factors related to the vehicles that operate over the track. Track gauge on moderately curved track can normally be set at the standard 56 ½ inches [1,435 millimeters] to accommodate common wheel gauges. As curves become sharper, more consideration should be given to ensure that sufficient freeplay is provided to prevent wheel set binding. Factors involved in this analysis are the radius of curve under consideration and wheel diameter, shape of the wheel flange, wheel gauge, and wheel set (axle) spacing on the light rail vehicle truck. Systems with mixed fleets and a variety of wheel and axle configurations must consider the ramifications associated with each and develop a compromise among the various requirements. Conventional wisdom suggests that track gauge must be widened in curved track; however, this axiom is largely based on railroad experience with extremely large diameter wheels and very long wheelbases. By contrast, transit vehicles with smaller diameter wheels, short and narrow flanges, and short wheelbase trucks (i.e., axles are closer together) will often require no track gauge widening in curved track. Transit equipment may, therefore, require track gauge widening only on the most severely curved track segments and then only if the axle spacings, wheel flanges, and wheel diameters are large. Some equipment may need no track gauge widening at all, even at an 82-foot [25-meter] radius. As a guideline, it is recommended that systems that have numerous sharp curves select vehicles with shorter wheelbase trucks. Truck designs built with axles spaced 1800 to 1900 mm [about 71 to 75 inches] are generally satisfactory for universal use. For trucks with wheel diameters less than 28 inches [711 millimeters] and axle spacing less than 74.80 inches [1900 millimeters], gauge increase will not be required even if AAR wheel flanges are used. Trucks with small diameter wheels and short axle spacings can also negotiate extremely small radius curves as low as 36 feet [11 meters] with only slight widening, usually about ¼ inch [5 mm]. Conversely, large diameter wheels, large flanges, and long wheelbases will require gauge widening at appreciably greater curve radii than smaller trucks. Trucks with large diameter wheels and a long wheelbase will generally have unsatisfactory operation on extremely sharp radius curves, are typically limited to curve radii of at least 82 feet [25 meters], and may require gauge widening on curves with radii less than 197 feet [60 meters]. If large, railroad-type wheel flanges are used in combination with narrow flangeway groove rails, even small track gauge increases are usually not possible because the gauge widening exacerbates the problem of back-to-back wheel binding. Reduction, rather than widening of track gauge in curved track has been considered on several systems in Europe and by at least one agency in North America as a way to improve vehicle-

Track Structure Design 4-11 tracking performance when passing through reduced radius curves using groove rail. It is thought that reduction of track gauge could also reduce wheel squeal by limiting lateral wheel slip, which is believed to be a main source of such noise. See Chapter 9, Article 9.2.3 for additional discussion on this topic. 4.2.4 Curved Track Gauge Analysis Requisite track gauge and flangeway dimensions in curved track must be determined analytically for each combination of vehicle truck factors. There are several graphical methods for analyzing this issue. The articles below will discuss two. The first, the Filkins-Wharton method, dates to the early 20th century. The second, a method known as the Nytram Plot, builds upon the Filkins- Wharton method. 4.2.4.1 Filkins-Wharton Flangeway Analysis The tight wheel-to-track-gauge freeplay and small wheel flange profiles that were common on traditional street railways allowed for smaller flangeways than those needed for railroad service. Hence, girder rails that were rolled specifically for streetcar systems had narrower flangeways than the flangeways sometimes used by steam railroads. (Steam railroads often had embedded/paved track in urban warehouse and wharf districts and several designs of girder rails were once rolled specifically for that purpose.) The narrower flangeways of the girder rails designed for streetcar service were more conducive in areas with pedestrian traffic. Mr. Victor Angerer was a Vice President of Wm. Wharton & Sons, a Philadelphia firm that was one of the leading special trackwork manufacturers of the early 20th century. In a paper presented before the Keystone Railway Club in 1913 and later reprinted in the Electric Railway Handbook,[1] Mr. Angerer said: …theoretically for track laid to true gage every combination of radius of curve and wheel base of truck, with a given wheel flange, calls for a specific width of groove to make the inside of the flange of the inside wheel bear against the guard and keep the flange of the outside wheel from grinding against the gage-line and possibly mounting it. It is manifestly impracticable to provide guard rails with such a variety of grooves or to change the grooves of the rolled rail. The usual minimum of 1-9/16 inch is wide enough to pass the AREA standard flanges on a 6-foot wheel base down to about a 45-foot radius, and the maximum width of 1-11/16 inches down to about a 35-foot radius. On curves of larger radius the excess width should be compensated for by a corresponding widening of the gage. If the groove in the rolled rail is too narrow for given conditions, it must be widened by planing on the head side of the inside rail, to preserve the full thickness of the guard, and on the guard side of the outside rail to preserve the full head. Unusual wheel bases such as 8 feet or 9 feet may require widening of the gage on some curves. This widening of gage is necessary only to bring the guard into play when the groove is too wide for some one combination of wheel and flange. In T-rail curves the guard is formed of a rolled shaped guard, or a flat steel bar, bolted to the rail. In special work and curves in high T-rail track a girder guardrail is often used. This is desirable, as it gives the solid guard in one piece with the running rail. The idea that a separate guard can be renewed when it is worn out does not work out in practice, as it

Track Design Handbook for Light Rail Transit, Second Edition 4-12 is usually the case that when the guard is worn the running rail is also worn to such an extent that it will soon have to come out also.[1] This excerpt provides still timely guidance in determining flangeway requirements, particularly for design of restraining rail systems, and evaluating the possible use of presently available groove rails. Around 1910, one of Mr. Angerer’s employees, Mr. Claude W.L. Filkins, developed a graphical technique for determining the optimum track gauge and flangeway dimensions for any given conditions of truck dimensions, wheel diameters, and wheel profile. The Filkins-Wharton diagram analysis was a simple and effective technique to establish the minimum flangeway openings required to suit wheel flange profiles, curve radii, and axle spacings. The following describes the Filkins-Wharton diagram procedures.[1] Figure 4.2.4 represents an AAR-1B wheel with a diameter of 28 inches [711 mm] placed on 115 RE rail on an 82-foot [25-meter] radius curve. In the illustration, the wheel is adjacent to the rail gauge line. On a conventional, rigid, non-steerable truck, the flange will never be sitting perpendicular to the curve radius but rather at a skew. That skew will vary in proportion to the wheelbase (distance between axles) of the truck, with longer wheelbases resulting in larger skews. In the example, the wheelbase is 72 inches [1828 millimeters]. Line A-B is the horizontal cut plane passing through the AAR-1B wheel profile [W] resting on the 115 RE rail head [R]. C-D-E represents a sectional view of the wheel at the plane defined by the top surfaces of the two rails. The line C-D-E is perpendicular to the axle. While the rail is actually curved, the length of rail head adjacent to section C-D-E is short enough to be considered a straight line. The line F-G represents a perpendicular line to the radius line and forms an intersecting angle of 2.0368 degrees to the wheel axis C-D-E. For a static condition, all four wheels will produce an approximately similar angle for line F-G using the combination of curve radius and wheelbase. (In practice, this is not the case for a rolling truck because it will always be skewed to the track in the opposite direction from the curve.) Geometric construction is applied to project the resulting flange profile on the plane H-J. Plane H-J is perpendicular to the rail head and radial to the curve. Projecting the points of the wheel in plan along the track arc to line H-J produces the outline K-L-M. Note that this shape is not the same as the wheel flange profile because the graphical exercise above is considering the entire space occupied by the flange below top of rail, including consideration of the angle by which the axles (and hence the wheels) are skewed to a radial line. In effect, the flange has been “fattened” to account for that skew. Outline K-L-M therefore represents the absolute minimum flangeway shape required to permit a vehicle truck with an AAR-1B wheel profile and the stated wheel diameter and wheelbase to negotiate the stated track radius. Track designers back in the early 20th century could then consult catalogs of available girder rails and select one which provided a flangeway at least that large. Naturally, additional flangeway clearance is still required to allow relatively free movement and to compensate for tolerances in the wheel mountings, wheel profiles, and track gauge tolerances—resulting in a wider actual flangeway. Flangeway depth must consider wheel tread wear and special trackwork design features as flange-bearing flangeways.

Track Structure Design 4-13 Figure 4.2.4 Filkins-Wharton diagram for determining flangeway widths

Track Design Handbook for Light Rail Transit, Second Edition 4-14 Figure 4.2.5 illustrates the flangeway requirements using outline K-L-M considering both flangeways using 59R2 groove rail and standard track gauge and AAR wheel gauge. Note how the “fattened” wheel flanges just barely fit in the flangeway. Any appreciable amount of wheel flange wear would allow the wheelset to shift laterally, resulting in contact between the back of the wheel and the tram of the 59R2 groove rail on the opposite rail. That condition will be discussed further in Article 4.3 of this chapter. Figure 4.2.5 Filkins-Wharton plot to establish flangeways See Chapter 5 for additional guidance concerning maximum flangeway width in embedded track and railway/highway crossings. 4.2.4.2 Nytram Plots—Truck-Axle-Wheel Positioning on Curved Track Claude Filkins was limited to manual drafting methods and the accuracy of Filkins-Wharton diagrams was therefore limited when using drawing sheets of practical dimensions. Filkins- Wharton diagrams produced manually were forced to graphically shrink track gauge and wheelbase in order to depict an entire truck assembly on a reasonably sized drafting sheet. The method also does not consider dynamic truck behavior, but presumes the truck is always square to the track. A modified version of the Filkins-Wharton diagram, referred to herein as the Nytram plot, has therefore been developed taking advantage of the power of computer-aided design and drafting (CADD) as an analytical tool. CADD provides the track designer with the ability to develop a full- sized picture of the entire vehicle truck positioned on a curved track, including rotation of the truck to mimic actual behavior. These CADD images can then either be plotted at reduced scale, or selected portions of the diagram can be printed at full size.

Track Structure Design 4-15 To illustrate the methods involved, a series of figures have been developed that illustrate the fundamentals of adapting track gauge to wheel gauge, wheel contour, and positioning of a truck on a segment of curved track. The figures consider the following parameters: Wheel Profile Modified AAR-1B 5 ¼ inches [133 millimeters] overall width Wheel Diameter 28 inch [711 millimeters] Wheel Gauge 55.6875 inches [1414.5 millimeters] (AAR standard) Wheel Back to Back 53 3/8 inches [1356 mm] (AAR standard) Axle Spacings 74.80 inches [1900 millimeters] Curve Radii 82 feet [25 m], 300 feet [91.4 m] and 600 feet [182.9 m] An AAR wheel profile and gauge has been used in the examples so that the variables are limited to curve radius. Projects that wish to use groove rails with narrow flangeways need to consider transit profile wheels with narrow flanges and wider back-to-back wheel gauge. 4.2.4.2.1 Nytram Plot—Wheel Profile Sections The first step in developing a Nytram plot is to take sections of the wheel at several elevations at, above, and below top of rail. Figures 4.2.6 and 4.2.7 show horizontal sections of a selected wheel profile that have been derived at the gauge line elevation, at the top of rail, and, where appropriate, at the top of a restraining rail positioned 3/4 inches [19 millimeters] above the top of the running rails. If a restraining rail is present at a different elevation, a different section would obviously be required. Note how the length of each section (parallel to the rail) is dependent on the diameter of the wheel. Large diameter wheels will have longer wheel sections and will occupy more space in the flangeway, especially in curves. LRT systems with mixed vehicle fleets with wheels of varying diameters will need to consider each wheel separately. Figure 4.2.6 Wheel sections for Nytram plot—oblique view

Track Design Handbook for Light Rail Transit, Second Edition 4-16 Figure 4.2.7 Wheel sections for Nytram plot—modified AAR-1B transit wheel

Track Structure Design 4-17 Figure 4.2.7 illustrates the details of the process. Identification points are established on the surface of the wheel flange to define points of horizontal flange sections and assigned numbers from zero up to 10. Those points also define circular arcs in 1/8-inch [3.175-mm] increments along the wheel surface, as seen in the elevation view of the wheel on the left side of the figure. Projecting points 0 to 9 from both sections as shown, a horizontal section or “footprint” of the wheel can be developed at various heights above or below the top-of-rail elevation. Using these wheel sections, the actual positions of the vehicle truck axles and wheels can be superimposed on a section of curved track of any specific radius so as to simulate the complete truck in a skewed position. This allows the designer to determine the maximum “angle of attack” of the leading wheel with the outer rail, the points of wheel flange contact with both running rails and the restraining rail (if present), and the wheel flange-to-rail clearances. It will also determine whether any wheel binding will be present should the track gauge be too tight. 4.2.4.2.2 Nytram Plots—Static Condition The next step is to graphically “assemble” a complete vehicle truck by mounting the wheel profile sections on imaginary axles and positioning those axles the correct distance from each other. That assembly is then positioned on a graphical representation of the track drawn to scale at the curve radius of interest, perpendicular to the radius line and with the flanges all equidistant from the two rails. Figure 4.2.8 illustrates a stationary transit vehicle truck with a 28-inch [711-mm] diameter wheel, AAR wheel gauge, and an axle spacing of 74.80 inches [1900 mm] positioned on an 82-foot [25- meter] radius curve. This figure was developed by following these steps: • Develop three curve centerlines using radii of 82, 300, and 600 feet [25, 91.44, and 182.88 meters, respectively]. (Figure 4.2.8 is actually drawn as an 82-foot/25 meter radius curve so as to more clearly illustrate the conditions. The calculated dimensions for the other radii have been added to the graphic for comparison purposes.) • Develop the track gauge lines concentric with the track centerline. In this exercise, standard 56.5-inch [1435-millimeter] track gauge has been used. • Develop the vehicle truck centerline perpendicular to the track radius line, measuring half the axle spacing in each direction, and placing the center of each axle on the centerline of track. • Develop the truck axles perpendicular to the centerline of the truck. • Place the vehicle wheel sections developed in Figure 4.2.7 on the axles spaced at the back-to-back distance perpendicular to the axle centerline. The truck should now be centered on and square to the track. • To establish wheel flange clearances to the gauge line of the track, graphically measure the distances from the gauge line of the rail to the closest point on the wheel profile outline at gauge line elevation. These measured dimensions are normal to the rail but not parallel to the axles. Note that, because of the skew of the truck, these clearances will always be less than the wheel-gauge-to- track-gauge freeplay that will exist on tangent track. Note also that these clearance dimensions

Track Design Handbook for Light Rail Transit, Second Edition 4-18 vary depending on whether the measurement is done at gauge line elevation or at some other elevation at or below the top-of-rail plane. When considering modern rail sections with compound radius gauge corners paired with a conformal wheel profile, a precise evaluation will typically reveal that the first point of contact between wheel and rail occurs at a point about 3/8-inch [9.5- mm] beyond the gauge line and not at the gauge face of the rail. However, this distinction can generally be neglected for ordinary analysis. Figure 4.2.8 Static Nytram plot Similar plots (not shown here) were undertaken with the same truck parameters for curves with 300-foot [91.44-meter] and 600-foot [182.88-meter] radii, and the clearance results were added to Figure 4.2.8. The intersection angles between the perpendicular truck and the tangent point to the track arc have been determined graphically and are shown for the three curve radii for comparison. The above Nytram description and illustration depicts a static truck superimposed on a curve perpendicular to the radius line so as to illustrate the basic concepts. To determine the operational flangeway widths and the angle of attack between the wheels and the rails, the actual dynamic truck skewing must be considered, as described below. 4.2.4.2.3 Nytram Plots—Dynamic Condition As a next step, so as to simulate the steering action of the vehicle truck traversing through the various curves, a set of drawings with the same truck parameters as above has been developed. These next figures simulate the typical steering action that occurs when a truck leaves tangent track and enters a curve. The leading outside wheel on the truck encounters the curved outside rail resulting in steering or deflecting of the lead axle and the truck.

Track Structure Design 4-19 Figure 4.2.9 illustrates the same vehicle truck as shown in Figure 4.2.8. The truck has been rotated about the center of the truck (Point “A”) in a direction opposite that of the curve in the track until a first point of contact is found between a wheel and a rail. For track without restraining rail, this first point of contact will always be at the wheel on the leading axle that is riding on the outer rail of the curve, here designated as Wheel “B.” This mimics the condition that occurs when a truck first enters a curve from a segment of tangent track. Figure 4.2.9 Nytram plot—rotated to first point of contact As a point of order, it should be noted that the truck does not actually instantaneously rotate about Point “A” at the beginning of a curve. The rotation shown in Figure 4.2.9 is merely a graphical tool for approximating the net effect of a series of events. Those events include an initial wheel contact as the truck continues to roll straight for a short distance into the curve until initial flange contact is realized. Concurrently, the effects of differential rolling radius on conical wheel treads will be felt due to the shorter rolling distance along the inner rail. In combination, these events have the same net effect as the graphical truck rotation. Figure 4.2.10 illustrates the next step. Once the leading outside wheel initially contacts the outer rail, the rolling wheel along the inner rail (which has a shorter distance to travel) causes the truck to continue to rotate, seeking a second wheel-flange-to-rail contact point. However, this additional rotation will not occur about Point “A,” but rather about that first point of contact at Wheel “B,” as identified in Figure 4.2.9. Typically, the second point of contact occurs at the inner wheel of the trailing axle (Wheel “D”); however, trucks with moderate self-steering capability may not encounter the second contact point. With the truck in this fully rotated condition, it is then possible to graphically measure various parameters including • The angle of attack of the lead wheel to the outside running rail. • The wheel-flange-to-rail clearances at each wheel.

Track Design Handbook for Light Rail Transit, Second Edition 4-20 • The absolute minimum flangeway widths necessary to permit free passage of the flanges. The last bullet point above becomes an important issue in embedded track using tee rail because if the flangeways are too narrow, unintentional contact could occur between the backs of the wheels and the paving material that defines the edge of the flangeway. It is necessary to add a factor to that dimension to account for construction/fabrication tolerances and also to allow some running clearance. Figure 4.2.10 Nytram plot—rotated to second point of contact It is important to note that the analysis above is considering an idealized condition where both the wheels and rails are new and unworn and the track gauge has been constructed with a zero tolerance. Worn wheels and rails and wide track gauge will result in larger angles of attack and larger values of wheel-flange-to-rail clearance. Because of these issues, wider flangeways than the dimensions determined will always be required in plain, non-guarded track. This type of interface study should be undertaken jointly by the project’s vehicle and track designers. Incorporation of factors to account for peculiarities of the truck design, as identified by the vehicle engineers, may be appropriate. For example, the Nytram drawings presume that the truck remains absolutely rectilinear and do not account for either potential axle swivel that might be permitted by a flexible primary suspension system at the journals or any possible twisting or racking of the vehicle truck into a parallelogram configuration. These conditions may vary in each manufacturer’s truck design. 4.2.4.2.4 Nytram Plots Considering Restraining Rail The drawings as developed above do not consider restraining rail; however, a measured inside rail flangeway width has been stated on the drawings as a reference. If the use of restraining rail

Track Structure Design 4-21 is selected on a system due to restricted sharp radius curves, then a similar scenario should be undertaken using the parameters of the vehicle truck and track system to establish the flangeway. Figure 4.2.11 illustrates the truck shown in Figures 4.2.8 through 4.2.10 statically mounted on a curve with a restraining rail mounted along the inside rail of the curve. For purposes of this trial, the restraining rail has been positioned flush with the top of the running rails, and the flangeway width has been set at 2 inches [51 mm]. Figure 4.2.11 Static Nytram plot with restraining rail Figure 4.2.12 shows the truck rotated to the first point of contact, which now occurs not at Wheel “B” but rather between the restraining rail and the back face of Wheel “C.” In Figure 4.2.13, the truck is rotated about that first point of contact at Wheel “C” to find the second contact point. For curves that do not have a restraining rail on the outer rail, that second point of contact will still occur at Wheel “D.” Clearances and angles can then be measured graphically as previously discussed. Note how, just as the calculated clearances will vary depending on track gauge, the width of the flangeway is critical as well. If the flangeway is wide, the first point of contact may still occur at Wheel “B.” In such cases, the restraining rail may not come into play until a combination of wheel flange wear and rail gauge face wear results in some vehicle trucks contacting the restraining rail at Wheel “C.” For extremely sharp radius curves using double restraining rails, the same procedures are required to establish both flangeway widths.

Track Design Handbook for Light Rail Transit, Second Edition 4-22 Figure 4.2.12 Nytram plot with restraining rail—rotated to first point of contact Figure 4.2.13 Nytram plot with restraining rail—rotated to second point of contact

Track Structure Design 4-23 All of the illustrations above use AAR wheel profiles and wheel gauge. If the same analysis is performed using a transit wheel profile and/or wheel gauge, different values will ensue. In general, the use of AAR parameters, particularly their wheel mounting gauge, requires a wider flangeway than when using transit wheel parameters. As a guideline, it is recommended that the inside restraining rail flangeway width be set to provide shared contact so that the inside back face of the wheel makes contact with the restraining rail face while the outside wheel is simultaneously contacting the gauge corner of the outside rail. This will theoretically divide the lateral steering force between both wheels and rails. However, the following conditions must be recognized: • The ability to precisely achieve dual contact must be tempered by the practical fabrication dimensions and tolerances. It is impractical to specify flangeway widths to a fabrication tolerance finer than +/- 1/16 inch [1.6 millimeters]. • In practice, simultaneous dual contact may not occur immediately; however, wear at either the gauge face of the outside running rail or the working face of the restraining rail will eventually lead to routine shared contact. • On any LRT system of appreciable size, variations in wheel wear on various cars plus variations in construction and maintenance tolerances of the track at any given location will guarantee that no two cars will track through any particular curve exactly the same way. Some vehicles will end up always being steered solely by the high rail. Other vehicles might have 100% of their steering via the low restraining rail. Some vehicles may result in lateral load sharing, but it will rarely be a 50-50 split and will likely fluctuate with variations in the dynamic track gauge because of tolerances and railhead deflection. • Some small amount of lateral load will be transferred via top-of-rail friction; however, that will be erratic since wheel slip—both lateral and longitudinal—is essential during negotiation of tight radius curves. It is not a perfect system where loadings can consistently be predicted with mathematical and mechanical precision. In spite of some shortcomings, the Nytram plot concept described above has been used on many projects with appreciable success. By careful trial analysis, varying the parameters, an optimum configuration can be derived. In general, keeping the truck as close as possible to being square to the track will result in the optimal long-term performance. See Article 4.3 of this chapter for a discussion of restraining rail, including pros and cons regarding its use. Even if no restraining rails are used, the Nytram plot is a useful tool for identifying the minimum flangeways necessary in curved embedded track. On more than one occasion, the flangeway formed in the pavement next to sharply curved, embedded tee rails has been discovered to be too narrow when the back sides of the wheels began grinding into the concrete. 4.2.5 Rail Cant and Wheel Taper—Implications for Track Gauge Rail cant is a significant factor in wheel-to-rail interface. Cant describes the rotation of the rail head toward the track centerline. It is intended to complement conical wheel treads in promoting

Track Design Handbook for Light Rail Transit, Second Edition 4-24 self-steering of wheel sets through curves. The cant also moves the vertical wheel loading away from the gauge corner of the rail and toward the center of the ball of the rail head. Tee rails are generally installed at 1:40 cant in both tangent and curved track. Additional rail cant could be considered at short radii embedded curves below 300 feet [91 meters] at the low rail. This additional cant can be applied by installation of a 1:30 cant shim at the time of construction. The additional cant design duplicates the asymmetrical offset rail head grinding of the low rail, retaining the true crown profile of the rail head. This procedure has been used to reduce wheel squeal on at least one LRT system with favorable results. Zero cant is usually specified through special trackwork so as to simplify the design and fabrication of trackwork components. Canted special trackwork is now often specified for high- speed rail operations, but there is little benefit for doing this at the relatively low speeds commonly reached on LRT operations. When using tee rail, rail cant is achieved by using one of the following: • Concrete cross ties with the rail cant cast into the rail seats. • Canted tie plates on timber cross ties. • Canted direct fixation rail fasteners on a flat concrete invert. • Flat direct fixation rail fasteners on a canted concrete invert. In embedded track, the rail cant can be incorporated into the gauge ties that are usually used to hold the rails. Modern groove rails such as 59R2, which effectively incorporate cant into the rolled head and can therefore be laid on flat fasteners, are preferable to the older designs (such as 59R1), which must be placed on canted fasteners if cant is desired. 4.2.5.1 Tapered Wheel Tread Rationale Both railroad and the majority of transit wheel tread designs are typically tapered to be shaped like a truncated cone. A cone that is lying on a flat surface will not roll in a straight line. But a pair of conical wheels that are rigidly mounted on a solid axle, each supported on a single edge—such as at each rail—can be made to follow a straight path provided the axle axis is held rigidly at right angles to the direction of travel. Railway wheel design takes advantage of this geometric relationship to facilitate self-steering of trucks through gentle curves without requiring interaction between the gauge side of the high rail head and the wheel flanges. The usual conicity of the wheel tread is a ratio of 1:20. This results in a wheel that has an appreciably greater circumference close to the flange than it has on the outer edge of the wheel tread. In curved track, this differential moderately compensates for the fact that the outer rail of a curve is longer than the inner rail over the same central angle. The wheel flange on the outer wheel of the leading axle of a conventional solid axle truck shifts toward the outer rail when negotiating a curve; hence, that wheel rolls on a larger circumference. Meanwhile, the inner wheel flange shifts away from that rail and that wheel rolls on a smaller circumference. Thus, the outer wheel will travel forward a greater distance than the wheel on the inner rail even though they are both rigidly attached to a common axle and hence have the same angular velocity. As a

Track Structure Design 4-25 result, the axle assembly steers itself around the curve just as a cone rolls in a circle on a table top. Note that rolling radius differential is maximized when the wheel and axle set is free to shift laterally an appreciable amount. An actual cone has a fixed slope ratio; hence, it can smoothly follow only one horizontal radius. A wheel and axle set with tapered wheels, on the other hand, can assume the form of a cone with a variable side slope by shifting the freeplay left and right between the wheel flanges and the rails. Hence, larger values of track gauge to wheel-gauge freeplay can be beneficial in that regard. However, that larger freeplay also allows significant truck skewing and increases the angle of attack between the leading wheel and the outer rail of sharp curves. Railroad wheel sets mounted at AAR standard wheel gauge and tapered at 1:20 theoretically eliminate flanging on curves with radii over 1900 feet [580 meters], which is about a 3-degree curve. Many railroad design criteria specify 3 degrees as the desirable maximum curvature. Below that radius, contact between the outside wheel flange throat and the gauge corner of the outside rail provides a portion of the steering action. Nevertheless, tapered wheels still provide a significant degree of truck self-steering that reduces flanging on curves with radii as small as 328 feet [100 meters]. For sharper curves, flanging is the primary steering mechanism. However, wheel sets that have reduced freeplay between wheel gauge and track gauge will commence flanging at a higher curve radius than a wheel set using AAR wheel gauge. Therefore, transit wheels self-steer only on relatively large radii curves, due to the fact that the reduced freeplay between wheel gauge and track gauge allows only very limited differential rolling radii on a conical wheel before the wheel begins flange throat contact with the gauge corner of the rail. Wheel profiles that have a cylindrical tread surface do not self-steer through curves of any radius; hence, flanging is the primary steering mechanism. Conical wheels that are not re-trued regularly also lose their steering characteristics because the contact patch becomes excessively wide as a significant portion of the wheel tread matches the contour of the rail head. Hollow worn wheels develop a “false flange” on the outer portion of the tread and can actually attempt to steer the wrong way as the rolling radius on the tip of the false flange can be equal to or greater than the rolling radius on the flange-to-tread fillet. The importance of a regular wheel truing program cannot be overstated, and track designers should insist that vehicle maintenance manuals require wheel truing on a frequent basis. The center trucks on 70% low-floor light rail vehicles do not have wheels rigidly mounted on a solid rotating axle. Instead, as described in Chapter 2, the center truck design consists of low- level “crank axles” providing independently rotating wheels (IRWs) mounted on stub axles. Since these pairs of wheels are not forced to have the same rotational velocity, these trucks derive no self-centering benefit from tapered wheels. They also behave differently in curves, and the steps described for Nytram plot truck rotation in Article 4.2.4 may not apply. For additional insight into low-floor car performance and design refer to TCRP Report 114: Center Truck Performance on Low-Floor Light Rail Vehicles.

Track Design Handbook for Light Rail Transit, Second Edition 4-26 4.2.5.2 Rail Grinding Rail grinding is essential in transit track maintenance and is discussed in Chapters 9 and 14 of this Handbook. However rail grinding can also play a role in new track design. Normal rail grinding developed around the needs of the freight and passenger railroad industries and is focused on removal of rail head defects with the objective of extending the service life of the rail. The usual rail-grinding operation on freight railroads involves • Removal of rail head corrugations. • Removal of top-of-rail and gauge corner defects, such as rolling contact fatigue checking, and provision of gauge corner relief to defer re-initiation of defects in both the top-of-rail and in the gauge corner. • Reshaping the top-of-head to a preferred rail head contour. The rail-grinding service industry has developed equipment and detailed procedures tailored to the needs of its railroad customers. However, the needs of a rail transit system are distinctly different than those of a railroad. Differences include the following: • Freight railroads need to have very long stretches of rail ground during relatively short work windows. Work windows on transit operations can be even shorter. • Freight railroad rail grinding can use relatively coarse grinding stones since the heavy wheel loads of freight equipment will quickly erase the “signature” grinding pattern or marks. However, the comparatively small wheel loads of rail transit can take a very long time to erase the signature grinding marks. In the meantime, the wheel/rail interface oscillations initiated by the coarse grinding marks can grow into new problems, including high-pitched noise and even new rail corrugations. • Rolling contact fatigue type rail defects, which are common under railroad loadings, generally do not occur under transit loadings because transit’s light wheel loads do not stress the rail steel anywhere near as much as freight loads. • Rail corrugation patterns in rail transit are appreciably different than those in freight track. • Transit systems typically need an initial rail grinding to remove mill scale and light rust so that signal circuits will shunt reliably. This is not a concern for freight railroads since the heavy axle loads will quickly wear away any such surface contamination. Noise that originates at the wheel/rail interface has always plagued rail transit systems, and the condition of the rail head surface is a major contributor to noise. Rail grinding is utilized to remove two principal categories of unwanted surface imperfections that are a source of the noise. These are • “Mill scale,” both from the original manufacturing rolling of the rail and subsequent heat treating processes. • Rail head corrugation formed during operation, which has proven to be a detriment and key source of noise. However, conventional rail-grinding practice in North America has evolved around the needs of the freight railroads, who generally have different concerns relative to rail imperfections and the

Track Structure Design 4-27 general quality of the finished grinding. For example, mill scale is completely a non-issue for freight railroads. Finish tolerances are also much less, in part because the high axle loads of freight equipment will quickly roll smooth any coarseness from the grinding stones. But these matters are very important when grinding transit rail since the light axle loads will not smooth out any discontinuities. Conventional freight railroad grinding methods produce two undesirable conditions: • Transverse rail-grinding signature score patterns gouged into the rail head. • A series of flat grinding facets across the rail head, each approximately ½ to 7/8 inch [13 to 19 millimeters] wide. Both of these conditions lead to wheel/rail noise. The latter can, under light rail transit loadings, result in an erratic longitudinal tracking pattern by the wheels. If rail transit grinding is not carefully controlled with respect to grinder pass speeds and number and size of cross head facets, the resulting rough conditions could result in significant wheel/rail noise. Transit rail grinding therefore must be far more carefully controlled than freight railroad grinding. The types of stones used, the grinding pass speed, and the width of the facets must all be carefully controlled. “Acoustic grinding,” providing a clean duplication of the original rail head profile without signature grinding marks and flat facets is therefore much preferred for transit service. It involves both finer grit grinding stones and additional passes so as to virtually eliminate the facets and more precisely achieve the desired rail head profile. With effort, it is possible to achieve a rail head surface finish within 0.5 mil [about 13 microns] of the theoretical rail head contour. For further discussion on rail grinding requirements and methodologies refer to Chapters 9 and 14. 4.2.5.3 Asymmetrical Rail Grinding The objective of rail grinding on railroads is usually to remove rail surface imperfections such as corrugation and rolling contact fatigue (RCF) defects. A relatively recent practice (since about 1990) has been rail grinding designed to alter the location of the wheel/rail contact band. By grinding an asymmetrical profile on the rail head and having distinctly different contact band locations along the high and low rails of a given curve, the location of the contact patch on the tapered wheel tread can be optimized, thereby changing the rolling radius of wheels on a common, rigid, fixed wheel/axle assembly. Given a specific wheel contour, a special grinding pattern can be created for each curve radius, thereby optimizing the ability of the leading axle of a truck to steer through that curve. However, on curves sharper than the self-steering radius, this benefit cannot be fully realized by the trailing axle since it will always follow a slightly different path than the leading axle. Asymmetrical grinding also cannot assist curving of trucks with stub axles and independent rotating wheels. 4.2.5.4 Variation of Rail Cant as a Tool for Enhancing Truck Steering Rail cant variation, as stated previously, can improve the rolling radius differential on standard rail head profiles in a manner similar to that achieved by asymmetrical rail grinding. Aside from the structural implications of loading the rail closer to or further from its vertical axis, greater or lesser amounts of cant can be beneficial by altering the location point on the tapered wheel tread that

Track Design Handbook for Light Rail Transit, Second Edition 4-28 contacts the rail. Installing rails with no cant creates a contact zone or wear strip that is close to the gauge corner of the rail. In rails installed with 1:40 or 1:20 cant, the contact patch progressively moves further away from the gauge corner of the rail. Note that the greater the rail cant (e.g., the smaller the second figure in the cant ratio), the smaller the rolling radius of a tapered wheel, which increases the self-steering effect when wheels shift to maximum off-center position. Figure 4.2.14 illustrates the theoretical contact patch locations measured from the vertical centerline of 115 RE rail with an 8-inch [203.2-mm] crown radius. The lateral distance between the contact patches for 1:40 and 1:20 cants is 0.20 inches [5.1 millimeters]. This shift results in a decrease in wheel circumference at the contact point of 0.062 inches [1.6 millimeters] for a wheel with a 1:20 taper. While this may appear to be insignificant, if the higher cant is applied to the inside rail, it will increase the amount of curvature the wheel set can negotiate without flanging by a significant amount. For example, a light rail wheel set at transit wheel gauge will flange at about a 4,000-foot [1220-meter] radius if both rails are at 1:40 cant. But, if the low rail is canted at 1:20 while the high rail remains at 1:40, then the threshold radius for flanging could drop to as low as about 2,500 feet [750 meters]. Note also that the difference between the center of the rail and the center of the contact patch will vary with the crown radius of the rail. Wheels running on rails with smaller crown radii, such as the 8-inch [203.2-mm] crown radius that AREMA introduced to 115 RE rail in 2009, will behave slightly differently from rails with flatter heads. Cant differential, in effect, mimics asymmetrical rail profile grinding. However, the application of increased cant at the low rail in curved track can be considered even if asymmetrical rail grinding is practiced. Construction issues that ensue from a decision to use differential cant include the following: • In ballasted tracks, any curves with non-standard cant will need to employ different concrete ties (or different tie plates on timber ties) than for tangent track. Further, the curve ties would have right- and left-hand orientations that would have to be carefully monitored during track construction. There would also be inventory issues associated with having several designs of cross ties (or tie plates) that probably will not look all that much different at first glance. • In direct fixation track, the different rail cant could be achieved when pouring the plinths or by placing tapered shims beneath the rail fasteners. Jigs for top-down construction that facilitate adjustments to the rail cant are available. Either approach would be vastly preferable to having several different types of rail fastener in the track system, particularly if the differences between the fasteners are not visually obvious at first glance. Simplification of maintenance inventory is greatly appreciated by maintainers and provides better assurance that the right product will be used at the right location. • Differential cant is relatively easy to achieve in embedded track. Either the ties can be fabricated with the ends canted, or tapered shims can be inserted between the ties and the base of rail. However, in the case of tracks built with groove rails (many of which incorporate normal cant into the head by design), actually inclining the rail will, in effect, lower the lip of the tram with respect to a plane defined by the tops of the two running rails. If the track design depends on the tram to act as a restraining rail, the tram will be less effective because it sits lower.

Track Structure Design 4-29 Figure 4.2.14 Rail cant design and wheel contact

Track Design Handbook for Light Rail Transit, Second Edition 4-30 The benefits of differential cant, like those of asymmetrical rail grinding, decline as the wheels and rail wear. As wheel treads wear toward a flat or hollow profile and rails wear to conform with the wheel profile, self-steering capabilities decline. Moreover, once the rail has worn, the contact patch will need to be restored to its as-designed location by asymmetrical rail profile grinding. Records must therefore carefully designate where zones of either asymmetrical grinding or differential cant exist so that future maintenance grinding operations can make adjustments to the angles of the grinding stones. The true benefit of using a tapered shim versus asymmetrical grinding during initial construction is the retention of the original rail head profile and specifically the crown radius. The desire in rail and wheel maintenance is to retain or reestablish the original rail profile contour and restore the designed wheel profile by precision rail grinding and wheel truing, respectively. 4.2.6 Construction and Maintenance Tolerances—Implications for Track Gauge The most precisely calculated standards for track gauge and flangeways will be of no value if the track is not constructed and maintained in a manner that ensures that the design intent is achieved in practice. Obviously, perfectly constructed and maintained tracks are not possible, and the cost of achieving such perfection would probably exceed the value of the benefits that would ensue. Accordingly, tolerances must be specified that both protect the design objective as closely as possible and are practical and achievable with the materials and equipment available. 4.2.6.1 Tolerances—General Discussion Tolerances for trackwork fall into four categories: • Manufacturing/Fabrication Tolerances: The rolling, casting, machining, and finishing tolerances of track materials need to be appropriate to the intended service condition. On a light rail transit project, in virtually all cases, track materials should be of the highest standard/quality for new materials, and tolerances will hence be tighter than those used for ordinary freight railroad track. Rarely are there any tracks on a light rail system where second-quality or used materials are appropriate. Manufacturing and fabrication errors in finished products are difficult (and sometimes impossible) to correct in the field and can place a burden on the installation contractor attempting to construct acceptable track with inferior materials. See Chapters 5, 6 and 13 for additional discussion on this matter. • Field Construction Tolerances: Track construction tolerances are most often specified with the use of new materials in mind. If used materials, such as relay grade rail, are employed, then construction tolerances may have to be less restrictive. • Field Maintenance Tolerances: These represent the acceptable limits of wear and track settlement or misalignment for track systems components. After components are worn to this level, performance is considered to be sufficiently degraded such that wear and deterioration are likely to occur at an accelerated rate. At that time, maintenance should be performed to restore the system to a condition as close as possible to its new, as- constructed state. • Field Safety Tolerances: These represent the levels beyond which the system is unsafe for operation at a given speed. The FRA Track Safety Standards are a well-known example. If track systems are permitted to degrade to an unsafe condition, performance

Track Structure Design 4-31 will be unsatisfactory, wear will be excessive, and the cost of restoration to a satisfactory state will be high. Either immediate corrective repairs, reduced speeds, or both are required once track has deteriorated to this condition. In all cases, the degree of uncertainty associated with the measurement methodology should be considered. It must also be recognized that the geometric parameters of the track under load can be appreciably different than when it is not loaded. 4.2.6.2 Tolerances and Track Gauge The reduced differential distance between track gauge and wheel gauge in transit systems governs the gauge tolerances for both. A suggested practice is to have a plus tolerance for track gauge and a minus (no plus) tolerance for wheel gauge, especially when the track gauge/wheel gauge freeplay is small by design. While both track gauge and wheel gauge typically have plus and minus tolerances, so as to avoid interference, the minus tolerance on track gauge and the plus tolerance on wheel gauge should be as close to zero as possible. If performance of the system is to be as expected, it is equally important that the vehicle side of the wheel/rail interface be built to very specific dimensions and within tight tolerances. Achieving tolerances on wheelsets in new light rail vehicles is rarely an issue. Where projects have come to grief at that interface, the fault usually lies in a lack of coordination between the vehicle engineer and the track engineer on issues of wheel profile and wheel gauge. 4.2.6.3 Suggested Track Construction Tolerances Transit track construction tolerances are more restrictive than conventional railroad standards. Table 4.2.1 lists suggested track construction tolerances for the three general types of LRT track construction. The following should be considered in developing tolerances to a particular project: • Achieving accurate track gauge when constructing with concrete cross ties is much easier than when constructing with timber ties as the former are manufactured with the rail fastening assemblies included. Strictly speaking, the tolerances given for concrete tie yard track are unnecessarily tight; nevertheless, they are easily achievable. • When considering minus tolerances for track gauge, consideration should be given to the freeplay between the track gauge and the wheels. The minus tolerance can be more liberal if AAR wheel gauge is used than if a transit wheel gauge is used. • The tolerances given are generally independent of train speed. Embedded and direct fixation tracks in slow speed secondary tracks (such as in a shop) can reasonably use looser tolerances. See Chapter 13, Article 13.2.3.4, for additional discussion concerning track construction tolerances.

Track Design Handbook for Light Rail Transit, Second Edition 4-32 Table 4.2.1 Track construction tolerances Construction Tolerances Location Tolerances Type of Track Track Gauge (5) Guard Rail Gauge (5) Cross Level (5) Horizontal Alignment Deviation (1) (5) Vertical Alignment Deviation (1) (5) Horizontal Alignment Variable (6) Vertical Alignment Variable (6) Ballasted concrete cross ties (Main Line) +/- 1/16” [+/-1 mm] +1/8”,-1/16” [+3,-1 mm] +/- 1/8” [+/- 3 mm] 1/4” [6 mm] (2) 1/4” [6 mm] (3) 1/2” [13 mm] 1/2” [13 mm] Ballasted timber cross ties (Main Line) +/- 1/8” [+/-3 mm] Ballasted concrete cross ties (Yard) +/- 1/16” [+/-1 mm] +1/8”,-1/16” [+3,-1 mm] +/- 3/16” [+/- 5 mm] 3/8” [9 mm] 3/8” [9 mm] 1/2” [13 mm] 1/2” [13 mm] Ballasted timber cross ties (Yard) +3/16”, -1/16” [+5, -1 mm] Direct Fixation +1/8”,-1/16” [+3, -1 mm] +1/8”,-1/16” [+3,-1 mm] +/- 1/8” [+/- 3 mm] 1/4” [6 mm] (2) 1/4” [6 mm] (3) 1/4” [6 mm] 1/4” [6 mm] Embedded +1/8”,-1/16” [+3, -1 mm] +1/8”,-1/16” [+3,-1 mm] +/- 1/8” [+/- 3 mm] 1/4” [6 mm] (2) 1/4” [6 mm] (3) (4) 1/4” [6 mm] 1/4” [6 mm] (1) Deviation is the allowable construction discrepancy between the standard theoretical designed track and the actual constructed track. (2) Deviation (horizontal) in station platform areas shall be: zero inches [millimeters] toward platform, 0.125 inches [3 millimeters] away from platform. (3) Deviation (vertical) in station platform areas shall be: plus 0, minus 0.25 inches [6 millimeters] or in conformity with current ADAAG requirements. (4) Deviation at top of rail to adjacent embedment surface shall be plus 0.25 inches [6 millimeters] minus 0. (5) Rate of change variations in gauge, horizontal alignment, vertical alignment, cross level, and track surface shall be limited to 0.125 inches [3 millimeters] per 15 feet [4.6 meters] of track. (6) Variable is the allowable construction discrepancy between the theoretical mathematized and the actual as-built locations of the track. Tracks adjacent to fixed structures shall consider the as-built tolerances of the structures. The data in Table 4.2.1 should not be confused with tolerances pertaining to track maintenance and track safety limits. Track maintenance limits that define allowable wear and surface conditions are not included in Table 4.2.1, as they should be developed with due consideration to the needs of a particular transit operating agency. 4.3 GUARDED CURVES AND RESTRAINING RAILS It is customary in North American light rail track design to provide a continuous guard rail or restraining rail through sharp radius curves. The term “restraining rail” will be used throughout

Track Structure Design 4-33 this Handbook so as to avoid any confusion with either the guard rails positioned opposite a frog or the “emergency guard rails” that are often positioned between the running rails on bridges. In addition to the discussion that follows here, readers are encouraged to consult two other documents on the topic of restraining rail that were produced by TCRP Project D-07—TCRP Research Results Digest 82: Use of Guard/Girder/Restraining Rails [7] and TCRP Report 71: Track-Related Research—Volume 7: Guidelines for Guard/Restraining Rail Installation.[8] An additional source of useful information is an article, “Testing Girder Rail on the MBTA” [9], a 2007 discussion in a web publication titled, Interface—The Journal of Rail/Wheel Interaction. 4.3.1 Functional Description In a typical LRT installation, the restraining rail is installed inside the gauge line of the curve’s low rail to provide a uniform flangeway. Restraining rail provides additional wheel steering action using the back face of the flange of the wheel that is riding on the inside rail of the curve. The inside wheel contact with the restraining rail takes some of the centrifugal force resulting from lateral acceleration and thereby reduces the lateral-over-vertical (L/V) forces of the outer wheel at the gauge corner of the outer rail. Depending on the curve radius and the truck factors, the flangeway is typically 1 ¼ to 2 inches [32 to 51 millimeters] wide. The working face of the restraining rail bears against the back side of the flange of the inside wheel, guiding it away from the centerline of track and reducing the lateral contact force between the outside wheel’s flange and the outer rail of the curve. This essentially divides the lateral force between two contact surfaces. Experience shows that this greatly reduces the rate of lateral wear on the high rail. (Curiously, the computer modeling that is the basis of TCRP Report 71, Volume 7, concluded that curves without restraining rail should have less wear than curves with restraining rail. As of 2011, this difference between theory and actual practice had not been reconciled.) Restraining rail also, by increasing the rolling resistance force applied along the inner rail, counteracts the tendency of the inner wheels to move ahead of their mates on the opposite end of the axle. This encourages backwards slippage of the inner wheels, rotates the truck in the direction of the curve and thereby reduces the angle of attack between the wheel flange and the outside rail. In all cases, the use of restraining rail in a curve will reduce the tendency of the leading outside wheel to climb the outer rail, thereby preventing possible derailments. 4.3.2 Theory TCRP Report 71, Volume 7, describes two restraining rail philosophies that TCRP Project D-7 investigated: • Philosophy I—“Shared Contact.” This configures the system (track gauge, wheel gauge, wheel profile, and flangeway width) so that simultaneous “shared” contact occurs between both the outer rail and the front of the flange riding that rail and the restraining rail and the back of the wheel riding on the inner rail. • Philosophy II—“No High Rail Contact.” This configures the system so that no contact occurs between the flange of the outer wheel and the gauge face of the outer rail. Effectively, all lateral loading and steering action occurs at the working face of the

Track Design Handbook for Light Rail Transit, Second Edition 4-34 restraining rail. (Note that a small amount of loading would still be carried by surface friction between the tops of the rails and the treads of the wheels, but that is so small that it is usually neglected.) A third philosophy, not directly addressed by TCRP Report 71, Volume 7, could be described as the following: • Philosophy III—“No Routine Restraining Rail Contact.” This configures the system so that routine contact occurs only between the flange of the outer wheel and the gauge face of the outer rail. The restraining rail is engaged only in the event that either (1) the outer wheel has begun to climb the outer rail or (2) the combination of wear on both the outer rail’s gauge face and the flange of the wheel allows contact to occur at the working face of the restraining rail due to the outward shift of the axle. Note that in the case of an incipient derailment, a Philosophy III restraining rail might be properly called a “guard rail” since, like the guard rail opposite a frog, it is engaged only when actually needed to prevent a mishap. Note that Philosophy I represents an idealized condition. It presumes that the system can be perfectly configured and ignores the realities of tolerances for fabrication, construction, and maintenance on both the track side and the vehicle side of the wheel/rail interface. Because of those factors, it is rarely possible to achieve Philosophy I during initial construction. Instead, what is usually done is to build the system so it more or less matches Philosophy II and allow it to “wear in” to a Philosophy I condition. This typically comes about through wear on the working face of the restraining rail. If track gauge is less than perfectly uniform, achieving equilibrium may require some wear on the gauge face of the outer running rail. Even once a shared contact condition is achieved, not all wheel sets will contact both the restraining rail and the outer running rail. Wheels with worn flanges will sometimes contact only the restraining rail while new wheel sets may contact only the outer running rail. Philosophy III is most commonly seen on European light rail operations, particularly those in Germany. The reason for this is contained in the German federal regulations concerning tramways and light rail transit, commonly known as “BOStrab.” BOStrab is a contracted form of Verordnung über den Bau und Betrieb der Straßenbahnen, which means Regulations on the Construction and Operation of Street Railways. BOStrab specifically prohibits the restraining rail configurations labeled above as Philosophy I and Philosophy II. The reasons for this restriction are unclear, but what is clear is that BOStrab is distinctly at odds with conventional North American practice in this matter. However, as is discussed in Chapter 2, Article 2.5.5.4, European rail vehicle designers and manufacturers, who are used to working under BOStrab, may object to the use of restraining rail, particularly if configured as per Philosophies I or II. Each of the options above has its adherents, but, for trackwork practitioners who prefer restraining rail, Philosophy I is the most popular. TCRP Report 71, Volume 7, concluded that Philosophy I does (on average at least) dramatically reduce lateral loading of the rails and hence can be instrumental in preventing flange climb derailments. Perhaps notable is the fact that TCRP Report 71, Volume 7, asserts that the lateral force exerted on the restraining rail under Philosophy II is greater than the lateral force that would be exerted on the outer running rail without any restraining rail. The reason for this difference is not clear. TCRP Report 71 also notes that lateral force on the outer rail is dramatically reduced (less than half) with shared

Track Structure Design 4-35 contact and that contact on only the restraining rail results in a higher lateral force than curves without restraining rail. This difference is unexplained but may be due to slight differences in the angles of attack between the restraining rail and the outer running rail. TCRP Report 71, Volume 7, states that the angle of attack is best with no restraining rail, but this assertion is counterintuitive and contrary to the results indicated by Nytram plots. Since TCRP Report 71’s simulation parameters for track gauge, wheel gauge, and flangeway width are unclear, this assertion requires more investigation. The work that led to TCRP Report 71, Volume 7, relied heavily on field testing that was performed at the Transportation Test Center in Pueblo, Colorado, in the early 1980s. Those tests were performed on the test center’s “Tight Turn Loop,” which has a 150-foot [45.7-meter] radius for a full 360 degrees of arc. Tests were conducted both with and without restraining rail using a then- experimental heavy rail transit vehicle known as the “State of the Art Car” (SOAC). Track gauge and the restraining rail flangeway were configured so as to match Philosophy II described above. Unfortunately, surviving documentation does not reveal several key parameters including the width of the restraining rail flangeways, the track gauge, the type of wheels on the SOAC, and other factors. What is clear is that the SOAC bears little resemblance to contemporary light rail vehicles, particularly low-floor and partial low-floor cars. The authors of this Handbook believe that additional instrumented field testing using contemporary light rail vehicles is appropriate and may well be essential to reconciling the differences between North American and European perspectives concerning restraining rail. 4.3.3 Application Criteria Restraining rails have been commonly applied on virtually all legacy rail transit systems (both light rail and heavy rail) in North America for well over a century. However, the thresholds at which restraining rails are applied varies greatly from system to system, Some transit agencies guard any curves with radii less than 1200 feet [365 meters], while others do not guard curves with radii larger than 300 feet [91 meters]. TCRP Report 71, Volume 7, recommends radius thresholds for restraining rail that vary depending on the type of vehicle and the track classification. Rather than condensing that information here, users of this Handbook are encouraged to scrutinize TCRP Report 71 in its entirety and make decisions based on the specific characteristics of their project. 4.3.3.1 Non-Quantifiable Considerations for Restraining Rail While designers are fond of exact criteria based on formulae, not all design can be that precise, and restraining rail is a key example of that. Non-quantifiable factors to consider with respect to the application of restraining rail are the following: • Lower train speeds reduce both lateral acceleration and the consequent lateral forces between the wheel flanges and the rail. This should reduce the lateral component of the L/V ratio, decreasing the probability of a wheel flange climb derailment. However, computer simulations conducted as a part of TCRP Project D-7 (results published in TCRP Report 71, Volume 7)[8] don’t fully support this premise. Nevertheless, field observations strongly suggest that unguarded curves of very tight radius can be safely operated at slow speeds while operation on the same curves at higher speeds presents a high risk for a flange climb derailment. Additional research, including field testing, is likely warranted.

Track Design Handbook for Light Rail Transit, Second Edition 4-36 • Frequently used tracks tend to develop a polish on the wheel/rail contact surfaces, which obviously reduces friction. Informal field observations suggest that tracks with these shiny rails that are both sharply curved and frequently used can be successfully operated without restraining rail and also with relatively little noise. By contrast, operation over rusty rail at the same curve radius can be very noisy and has a high probability of the wheel flange climbing the outer rail. TCRP Report 71, Volume 7, stipulates that no restraining rails are required if the coefficient of friction (µ) between the wheels and the rail can be kept under 0.4. (As a point of reference, the usual reference manual value for µ between clean, smooth, and non-lubricated steel surfaces is 0.8.) Rusty wheels or rails, wheels that have been freshly trued, or rails that have been freshly ground will result in higher values of µ. Lubrication of both the gauge face of the outer rail and the top of the inner rail of infrequently used (and hence rusty) sharp curves has been shown to decrease friction sufficiently to permit safe operation. • It is notable that the aforementioned experiment with the SOAC was performed well over a decade prior to the introduction of contemporary top-of-rail friction modifiers and about two decades before the introduction of modern on-board lubrication systems such as those described in Chapter 2, Article 2.8. Repeating the SOAC experiments with both modern LRVs and modern rail lubrication methods could produce substantially different results. • The research behind TCRP Report 71, Volume 7, concluded that restraining rail can prevent flange climb derailment that might otherwise occur because of track perturbations such as low joints and horizontal misalignments. In essence, poorly maintained track can derive more benefit from restraining rail than well-maintained track. It can therefore be inferred that rigid trackforms, such as embedded and direct fixation track, which are less likely to suffer misalignments, have less need for restraining rail than ballasted track. Moreover, the authors of this Handbook suspect that BOStrab’s prohibition of restraining rail is based in part on an expectation that track perturbations will never be allowed to reach the levels suggested in TCRP Report 71, Volume 7. In that regard, it must be noted that maintenance activities at European transit agencies are typically better funded than at transit authorities in the United States. • An LRT system using wheel flanges that are short, such as those that are common on legacy streetcar lines, will have a greater need for curve guarding than one that uses railroad-type wheels with tall flanges. This is because the lateral wheel loading is distributed over a narrower contact band along the side of the rail head thereby increasing contact stresses and resultant wear on both wheel flange and rail. Transit systems that use short flanges usually have a characteristic stepped wear pattern on the high rail of their curves. • Per TCRP Report 71, Volume 7, a rail vehicle wheel with an angle on the front face of the flange of less than 75 degrees will have a greater need for restraining rail than one with that optimal angle. • As of 2010, there did not appear to have been any studies of the optimal angle on the back face of the wheel where it interfaces with the restraining rail. Notably, the ATEA

Track Structure Design 4-37 standards recommended guard face angles that varied by the curve radius and most likely mimicked the angles to which restraining rails naturally wore in service. It should be noted that the contact point between an inside restraining rail and the back of the wheel usually occurs appreciably ahead of a vertical projection from the centerline of the axle. That location varies with both the curve radius and the angle of attack. • In theory, a system with vehicles that are equipped with a self-steering radial truck design should not need guarded track. • Conversely, LRT systems using LRV trucks with independently rotating wheels, which have no inherent steering capability, could possibly derive significant benefit from restraining rails since they can, if configured appropriately, correct extreme truck skewing through curves. Much of the conventional wisdom concerning restraining rails was developed nearly a century ago, when virtually all rail vehicles had solid, non-resilient wheels and solid axles. Resilient wheels and independently rotating wheels existed, but were experimental oddities. Now that both resilient wheels and independently rotating wheels are common, the authors believe that additional research is needed to optimize the wheel/rail interfaces where restraining rail is used, especially when used by vehicles using trucks equipped with modern suspension systems and wheels. Since there are dozens of modern truck designs and nearly as many designs of resilient wheels, a single set of criteria concerning restraining rail may not be possible. Track designers whose project includes a mixed vehicle fleet may need to consider the needs of each vehicle. 4.3.3.2 Longitudinal Limits for Restraining Rail Installations Curve guarding does not usually terminate at the point of tangency of a curve. Instead it extends some distance into the adjacent tangent track. This distance depends on a number of factors, including the resistance to yaw of the vehicle’s suspension system. The conservative designer will extend the restraining rail a distance no less than one axle spacing of a truck into the tangent track, typically rounded up to about 10 feet [3 meters]. When the curve is spiraled, the beneficial effects of guarding typically end long before the spiral-to-tangent location. In such cases, curve guarding can usually be terminated at the end of the spiral. Exceptions can be considered in cases where an unusually long spiral is used or when a compound curve condition exists with one curve segment guarded and the other not. The criteria for beginning curve guarding on the entry end of the curve are typically the same as for the exit end, accounting for the possibility of occasional reverse running train operation. As a guideline, the minimum guarding should begin at the tangent-to-spiral location of a spiraled curve so that the vehicle trucks are generally square to the track well before entering the portion of the spiral equal to the threshold radius for guarding. On some transit systems, the design criteria for projection of a restraining rail into adjoining tangent track are as much as three times the figures cited above. In at least one case, this was a direct reaction to problems encountered with derailments of a very early model of LRV in the 1970s. The actual cause of those derailments may have been that the truck’s equalization capability was not a match for the severity of the track twist, but extended restraining rail was the

Track Design Handbook for Light Rail Transit, Second Edition 4-38 nominal solution. More recent evidence suggests that no benefit is obtained by extending restraining rail more than about 10 feet [3 meters] into tangent track. 4.3.4 Curve Double Guarding Some transit agencies, notably the legacy systems, “double guard” extremely sharp curves, placing a restraining rail adjacent to the high rail as well as the low rail. These double-guarded installations are designed to counter the tendency of the second axle on a truck to drift toward the low rail. That motion occurs because the wheels on the inner rail, having a shorter distance to travel than those on the outer rail, are constantly moving ahead of their mates on the opposite end of each axle. As a result, the truck rotates in a direction opposite to the orientation of the curve, and the flange on the inside wheel of the trailing axle usually bears against the low rail. This brings the angle of attack on the outside wheel of the leading axle to a maximum. Depending on the stiffness of the journals of the trucks, the truck can actually assume the plan shape of a parallelogram. This condition is, of course, unstable, and something must slip to restore equilibrium. In a worst case, the leading outer wheel climbs the rail and derailment occurs, but the usual case is that the wheels on the inner rail skip backwards, thereby briefly rotating the truck in the direction of the curve. This backwards skipping is plainly visible on extremely tight curves, such as those that are common on legacy streetcar lines. This roll-skip-roll-skip process is repeated through the full length of the curve. Concurrent with the skip backwards is a lateral slip and rotation across the rail head at all four wheels as the truck rotates. This combined transverse and longitudinal slippage on the rail head—typically called “stick-slip”—is actually the source of most curving noise, especially on the low rail, where the slip distance is longer. In a double-guarded curve, the restraining rail placed alongside the outside rail prevents the truck from fully rotating to the point where the inner wheel on the trailing axle is in hard contact with the inner rail. Instead, the back of the trailing axle’s outer wheel is bearing on the outer restraining rail. This reduced truck rotation thereby reduces the angle of attack at the leading outside wheel. It also reduces the magnitude of each cycle of stick-slip, since the inner wheels don’t need to skip backwards as far to restore equilibrium. The outer restraining rail, by essentially pulling the trailing axle away from the inner rail, also keeps the truck reasonably square to the track, with both axles closer to a radial orientation, and assists in keeping the truck frame rectilinear. It also reduces the amount of forward motion that occurs before wheel/rail slippage occurs, effectively reducing the amplitude of each cycle of stick-slip. In superelevated, sharp radius curves where the vehicle speed is reduced, the vehicle truck may tend to hug and climb the low rail. The outer restraining rail reduces this wheel climb potential. As a guideline, a typical threshold for consideration of double-guarded track is for curves with radii of 100 to 125 feet [30 to 38 meters]. 4.3.5 Restraining Rail Design In North America, curve guarding on traditional street railway systems was most frequently achieved using a girder guard rail section somewhat similar to the 56R1 section illustrated in

Track Structure Design 4-39 Figure 5.2.5 of this Handbook, particularly for track embedded in pavement. For open track design, such as ballasted or direct fixation track, a separate restraining rail mounted alongside the running rail is more commonly used. The restraining rail can be machined from a section of standard tee rail, which can be mounted either vertically or horizontally. Specially rolled or fabricated steel shapes are also used, as described and illustrated in Chapter 5, Article 5.3. 4.3.5.1 Restraining Rail Working Face Angle Similar to the gauge face of the outer rail of a curve, the “working face” of a restraining rail on the inside of a curve tends to assume an angle to the vertical as it wears in. This is because the leading edge of the wheel, where it contacts the restraining rail, is non-tangential to the rail. The wheel also is contacting the restraining rail with two radial surfaces—the cross-sectional shape of the flange and the diameter of the wheel. This wear tends to stabilize at an angle of 10 to 15 degrees from the vertical, depending on the radius of the curve. The potential problem is that by the time the working face has reached the optimum angle, wear may have widened the flangeway some appreciable amount larger than its optimal dimension. For this reason, some restraining rail designs machine the working face of the guard at the time of fabrication, so no metal needs to be worn away before the optimal angle is reached. The former ATEA girder guard rails were manufactured with a 20-degree angle on the working face of the guard since that was optimal for the extremely tight minimum radii used on many legacy streetcar lines. The AREA girder guard rails last rolled in the United States in the 1980s had a 16-degree working face angle. By contrast, most European groove rails have an angle equivalent to roughly 9o30’. Notably, those ATEA and AREA guard face angles, which effectively are service-proven designs, may be more severe than a Nadal analysis would permit for gauge face wear on the outer rail of the curve. This dichotomy is a subject worthy of more detailed investigation. It generally is not necessary to consider a vertical angle on a restraining rail along the outer rail of the curve. 4.3.5.2 Restraining Rail Height Restraining rail designs typically also project above the plane of the running rails. The guards on American girder guard rails were ¼ to 3/8 inch [6 to 10 mm] above the top of rail. Some designs of separate restraining rails are as much as an inch above the running rails. The reason for this is to intersect more of the vertical back face of the wheel and not just the angled back of the flange. Restraining rails that project above the running rails also reduce any tendency of the wheel to climb the restraining rail. Notably, the lip on most European groove rails is typically 5 to 10 mm [0.2 to 0.4 inch] below the top of rail, which significantly reduces their possible effectiveness as a restraining rail. Restraining rails that project a substantial distance above the top of the running rails may interfere with some equipment on the light rail vehicle trucks, particularly magnetic track brakes. Elevated restraining rail positions can also interfere with hy-rail, rubber-tired, maintenance-of-way vehicles by lifting the rubber tires that propel the vehicle along the rail. This action may lift the hy- rail gear on the rear of the vehicle and result in a derailment. Some municipalities may object to elevated restraining rails in embedded track on the grounds that they might interfere with snow plowing; however, that is unlikely to actually cause problems unless the guard is significantly

Track Design Handbook for Light Rail Transit, Second Edition 4-40 more than the usual ¼ inch [6 mm] above the running rail, particularly if one looks objectively at the typical construction and maintenance tolerances for the pavement on urban streets. Both the height and the working face angle of the restraining rail should be considered when determining the most appropriate flangeway width. 4.3.5.3 ADAAG Considerations for Restraining Rail When restraining rails are used in a pedestrian path, care must be taken to comply with ADAAG requirements. This can restrict the height of the restraining rail above the running rail. Article 303 of ADAAG stipulates that the maximum permissible vertical bump in an accessible path is ¼ inch [6.4 mm]. An additional ¼ inch is acceptable provided it is ramped. While this requirement was most likely written with building doorsills in mind, it technically applies to any location along an accessible path, including crossing a railway track. Since there is no way to ramp across an open flangeway, this effectively limits the height of any restraining rail located along a pedestrian path to ¼ inch above the top of the running rail. However, vertical wear on the running rail must also be considered, since such wear would have the effect of increasing the height of the restraining rail. Viewed collectively, these considerations suggest that restraining rail in a pedestrian route should be no higher than level with the top of the new running rail. That way, once rail head wear occurs, the installation will still be in compliance with ADAAG. Note that the limitation stated above applies only to a designated pedestrian route, such as a crosswalk. Outside of such designated routes, the configuration of the restraining rail might be different, subject to any other constraints. For example, for track that is embedded in a mixed traffic lane of a public street, the restraining rail height should not present a hazard to vehicular traffic, especially bicycles and motorcycles. As noted above, the designer should consider the maximum wear condition when assessing the height difference. ADAAG restricts the width of rail transit system flangeways in an accessible path to no more than 2 ½ inches [63.5 mm]. Since that dimension is greater than any expected restraining rail flangeway, that requirement is not an issue. However, one topic that ADAAG does not address is flangeway depth. Small wheels on mobility aids can, when crossing a flangeway on a skew, easily spin and then drop down into the flangeway, possibly trapping the wheel. For this reason, it is strongly recommended that the restraining rail flangeways crossing an accessible path be no deeper than about 2 inches [50 mm] below the top of the running rail. If the restraining rail is machined from a vertically mounted tee rail, the open flangeway can be filled with an elastomeric grout up to the desired flangeway depth. This strategy also has the advantage of sealing the open flangeway, thereby excluding moisture that could penetrate and damage the track structure. 4.3.6 Omitting Restraining Rails—Pros and Cons The use of restraining rails is far from universal. Several of the light rail systems built in North America since 1980 use no restraining rail at all, even on the sharpest curves. In part, that can be explained by the fact that those systems were designed by persons with railroad trackwork backgrounds where restraining rails are virtually unknown. Nevertheless, those systems appear to function satisfactorily albeit with increased rail wear and slower operating speeds. It’s notable that if the flangeways on embedded tracks on those systems are not wide enough, the roadway

Track Structure Design 4-41 pavement can be abraded and damaged by the backs of the wheels as they briefly act as a de facto restraining rail. Arguably, in open trackforms (i.e., ballasted and direct fixation track) it may be both easier and more cost-effective to replace the high rail more often than it is to go to the extra expense and trouble of installing a restraining rail adjacent to the low rail. It has also been argued that the presence of a restraining rail can compromise the signal system’s ability to detect a broken running rail by providing an alternative path for the signal current. The details of the fastening system for a restraining rail also can be additional locations for stray current leakage. Due to BOStrab regulations, the use of restraining rail is uncommon on European LRT and tramway lines. Despite the near universal use of groove rail in European embedded track, open trackforms using tee rail on the same tramway lines will very frequently have no restraining rail on even the sharpest curves. Moreover, European tramways typically set track gauge very precisely so as to avoid any routine contact between the backside of the wheels and the lip on the groove rail, depending entirely on contact on the front of the wheel flanges for all steering action. Perhaps for this reason, most European groove rail sections have lips that are relatively thin. For example, the popular 59R2 section has a tram that is only 15-mm [0.59-inch] thick. The tram on the similar 60R2 section is only 21-mm [0.83-inch] thick. Those dimensions contrast sharply with the tram on the former ATEA’s girder guard rails, which was 1-15/32-inch [37-mm] thick. After girder rail was no longer rolled in the United States, several North American light rail systems began using the European groove rails. However, they did not yet appreciate the differences between the American and European designs and presumed the latter would perform in the same manner as the former. There was appreciable concern when it was first noticed that the lip on the European groove rails wore dangerously thin in a very short period. As will be discussed in Chapter 5, there are two European groove rail sections that provide a guard of appreciable heft; however, these guards are used by relatively few tramway systems compared to the more popular sections. Instead, following the letter of BOStrab, the general philosophy in most of Europe appears to be that the lip, or guard, on all groove rail sections is something that should only come into play when either the outer rail’s gauge face wear has reached a condemning limit or derailment is imminent. However, it should also be noted that, in general, standards for maintenance of light rail tracks are much higher in Europe than they are in North America. Moreover, European transport agencies are routinely provided with the budget necessary to both construct and maintain tracks to high standards, including replacement of worn rail. Few transit agencies in North America are so well funded. For this reason alone, direct comparisons between European and North American transit trackwork design principles— including application criteria for restraining rail—can be very misleading. As noted in Chapter 2, the general disuse of restraining rail in Europe has led European-based carbuilders to reduce the mass and stiffness of the vehicle axles in pursuit of reduction of unsprung vehicle mass. This reduces the vehicle’s capacity to accept the forces imposed by restraining rail contact. Vehicle engineers who are schooled in European practice may therefore strongly oppose the use of Philosophy I and II restraining rails. The track engineer wishing to use restraining rail on a new project may need to build a strong business case to justify the installation

Track Design Handbook for Light Rail Transit, Second Edition 4-42 on a life cycle cost basis, including vehicle-related procurement and operation and maintenance costs. 4.4 TRACK SUPPORT MODULUS Railway track acts as a structural element that undergoes stress and strain as a vehicle passes over it. The rails, rail fasteners or fastenings, cross ties, ballast, subballast, and subgrade are each a component of the track structure. Each undergoes some deflection as the wheel passes. The question of how the track structure reacts to wheel loads was studied as early as 1914, when a committee of what was then called the American Railway Engineering Association, chaired by Professor Arthur Newell Talbot of the University of Illinois, commenced investigations that led to the first definitive work on this subject. This Handbook provides sufficient information to design track; for additional reference, the designer is advised to study either the Talbot Reports of 1920 (available from AREMA in reprinted form) or Dr. William W. Hay’s textbook, Railroad Engineering, both of which provide more detailed explanations.[2],[5] Additional resources include AREMA’s Practical Guide to Railway Engineering. However, the reader is cautioned that engineering standards developed for freight railroad applications are frequently incompatible with the requirements of rail transit design, and direct application of information from these references should only be undertaken with due consideration of the differences in vehicles, loadings, and the trackway environment. Track modulus is an important subject, using complex mathematical calculations to analyze ballasted track as a structure. This analysis can determine appropriate rail weights, cross tie size, cross tie spacing, and ballast depth, as well as the need for subballast and any special subgrade preparation. Similar mathematical calculations can be undertaken for direct fixation and embedded trackforms. The track modulus factor value (typically represented by the symbol �) established in this article is a requirement of track design and one of the variables used in the calculations for ballasted track structural design (see Article 4.5.3) and direct fixation track structure design (see Article 4.6.3). In addition, track modulus is a parameter found in many of the calculations used by noise and vibration engineers when considering wheel impacts, contact separation, and vibration. 4.4.1 Modulus of Elasticity Ballasted track is often characterized as a beam supported on a continuous series of springs. Track modulus can be defined simply as the amount of deflection in these springs for a given wheel load. The greater the deflection, the lower the modulus. Conversely, a track with little deflection has a high modulus, which is generally considered important for ride quality and good serviceability in ballasted track. Most of the deflection in ballasted track results from deformation of the ballast and subgrade, with only minor deflections resulting from rail and cross tie compression. In order to minimize deflections, the track should have a deep section of well- compacted ballast and subballast with a sound, compacted, well-drained subgrade. This is crucial if total rail deflections for ballasted track are to be kept under the ¼-inch [6-millimeter] limit suggested by AREMA.

Track Structure Design 4-43 In direct fixation track, the track modulus is typically much higher, because the rail fasteners are made of elastomer with relatively high stiffness. In direct fixation track, the track designer is more frequently challenged to engineer a lower modulus into the track where possible, while still retaining required levels of gauge restraint and corrugation control. Reducing track modulus is desirable to the degree that it mitigates impact loading of the track and generation of high- frequency vibration. Soft direct fixation fasteners with elastomer in shear are available for providing a rail support modulus approximating that of ballasted track. However, as of 2010, such fasteners were somewhat more expensive than direct fixation fasteners of normal stiffness. The modulus of embedded trackforms is typically much higher than the modulus of open trackforms since there are very limited voids into which deflection can occur. When rails with elastomer/rubber boot encapsulation are embedded directly into concrete pavement or the bare rail is placed into other types of elastomeric embedment material, small but detectable amounts of deflection will result. However, it must be understood that elastomers cannot compress or deflect unless there is some void into which they can deflect. Solid elastomers are usually considered to be incompressible, and some amount of unloaded free surface area is required to allow the elastomer to deflect under load. The ratio of one of the loaded surfaces to the free surface is referred to as the “shape factor.” High shape factor produces high stiffness. The explanation below deals with ballasted track modulus, which can be determined using the this equation from Professor Talbot’s work:[5] P = -µy where P is the upward force on the rail per unit length µ is a factor determining the track stiffness or “modulus of track” given in units of pressure y is the vertical deflection measured at the base of rail The modulus of the track is defined as the vehicle load per unit length of track required to deflect the rail one unit. An example follows: Assume that on a track with cross tie spacing of 30 inches [762 mm], a wheel load of 20,000 pounds [88,964 newtons] causes a track vertical deflection of 0.375 inches [9.5 millimeters]. The force P required to deflect the track 1 inch [or 1 millimeter] is P20,000 � 1 0.375 � P 88,964 � 1 9.5 � P � 53,333 ���. ��.⁄ [P � 9,365 � ��⁄ ] The track modulus is equal to the force per unit of track length required to deflect the track by one unit, i.e., 1 inch [or 1 millimeter]. In this example, with cross tie spacing at 30 inches [762 millimeters], the track modulus is 53,333 30 � 1,778 �� ��⁄ �� � �⁄⁄ �9,365 762⁄ � 12.3 � ��⁄ �� � �⁄ �

Track Design Handbook for Light Rail Transit, Second Edition 4-44 The above analysis assumes that either the desired rail deflection is known or that maximum rail deflection is the primary criterion for the track design. Increasing the track modulus will dramatically reduce the bending moments in the rail. However, the higher modulus will also increase pressures on the ballast and subballast by directing more of the wheel load to the track support directly under the load. The ballast and subballast must be designed with the capacity to support those loads, as noted in the next section. Note that the variables used in calculating track modulus consider the support properties of a single rail and the loading of a single wheel, to simplify calculations. The load and deflection of a single rail applies equally to the track structure, since both the load and the stiffness are doubled. The effects of differential rail loading due to unbalance on curves are not specifically considered in the analysis but should be accounted for, along with impact, in determining worst-case service loads. 4.4.2 Track Stiffness and Modulus of Various Track Types The stiffness of rail, fastenings, and supporting structure determines the “Stiffness of Track,” whereas “track modulus” is concerned only with the support condition of the rail. (See TCRP Research Results Digest 79[6] for additional discussion on this point.) The types of track encountered on an LRT system—ballasted, direct fixation, and embedded—have a wide range of stiffness and track modulus because the components of each track substructure are dramatically different. Ballast provides the most flexible track structure support, while embedded track is usually the stiffest, with the highest track modulus value. Resilient direct fixation track can provide a wide range of stiffness by selection of rail fastener with engineered values of stiffness. 4.4.2.1 Ballasted Track The track modulus can be derived on a segment of existing ballasted track by measuring its deflection under load and calculating the modulus in accordance with the Talbot principles shown in Article 4.4.1. However, note that the Talbot formula is based upon track deflection due to a single axle load. If deflection is measured under a two-axle truck, an adjustment must be made because the nearby second wheel also contributes to local track deflection. Professor Arnold D. Kerr provides a method to adjust the modulus calculation to account for the weight of an adjoining second axle.[10] In many cases, the maximum rail deflection is not known or the maximum rail deflection is to be estimated for a given track structure that is yet to be built. The latter condition is frequently encountered in ballasted track design. The track modulus can be estimated considering the cross tie type and size, structure depth of subballast and ballast, type of ballast rock or stone, and the cross tie spacing. As a guideline, the track modulus with the track structure described can be expected to be in the following ranges: • 1500–2500 psi [10–17 N/mm2]: track - 18 inches [457 mm] depth of subballast and limestone ballast, timber ties spaced at 22 inches [558 mm]. • 2500–3500 psi [17–24 N/mm2]: track - 22 inches [558 mm] depth of well-compacted subballast and heavy stone ballast, timber ties spaced at 22 inches [558 mm].

Track Structure Design 4-45 • 3500–5000 psi [24–34 N/mm2]: track - 24 inches [609.6 mm] depth of well-compacted subballast and heavy granite ballast, timber ties spaced at 20.5 inches [520 mm]. • 5000–9000 psi [34–62 N/mm2]: track - 24 inches [609.6 mm] depth of well-compacted subballast and heavy granite ballast, concrete ties spaced at 28 inches [711 mm]. The type of fastening system between the cross ties and the rails can affect track stiffness, although apparently little research has occurred in that matter. Timber tie track using elastic rail clips will generally be stiffer than an otherwise identical track using traditional cut spikes. Track modulus has been known to vary and lose vertical support with an increase in applied load; that is, modulus under a 70-ton [63,500-kilogram] railroad freight car may have a lesser value when measured under a 100-ton [90,700-kilogram] railroad car. If this occurs, it is likely the result of overstressing the subgrade to the point that it deflects non-linearly. This is unlikely to occur under rail transit loadings except in cases where the subgrade soils are especially weak and compressible. A higher track modulus results in higher stress concentrations on the ties and ballast directly beneath the wheel than does a low-modulus track, which distributes more of the wheel load to adjoining ties. Concrete ties, which always increase the track modulus, therefore require a stronger ballast/subballast foundation than timber ties, and the track section must be designed accordingly. Provision of a stronger foundation generally entails a deeper ballast and subballast layer, installation of a geogrid, or other measures to distribute the load over a broader area of the subgrade. 4.4.2.2 Direct Fixation Track Unlike ballasted track, the track component deflections and elastic properties of direct fixation track are generally known. In direct fixation track, the vertical deflection occurs in the • Bending of the rail • Elastomer portion of the direct fixation fastener • Flexure of the direct fixation slab at the supporting subbase materials for at-grade installations. The track modulus of direct fixation track is determined by establishing the nominal spring rate of the elastomeric component of the direct fixation fastener. The spring rate is controlled by the ability of the elastomer to bulge, both along the free area at the periphery of the fastener (where it is usually exposed) and into the recesses within the fastener body. Manufacturers can control the spring rate within fairly narrow bands by customizing the sizes of these recesses, which are typically visible on the underside of the fastener body. Elastomer vertical static spring rates vary widely. Three popular spring rate ranges are • 50,000 to 80,000 lb/in [8,800 to 14,000 N/mm]—this range is highly resilient. • 90,000 to 140,000 lb/in [15,800 to 24,500 N/mm]—this range is standard. • 240,000 to 320,000 lb/in [42,000 to 56,000 N/mm]—this range is stiff.

Track Design Handbook for Light Rail Transit, Second Edition 4-46 It is worthwhile to note that the spring rate of a direct fixation rail fastener is virtually never linear from a condition of zero load up to maximum service load. Instead, due to the elastic behavior of elastomers under loading, a plot of load versus deflection would be a curve. The nominal spring rate of the fastener would be the slope of a line that is tangent to the load deflection curve within the zone of the actual service loading. Selection of the proper fastener stiffness should take into consideration wheel loads, fastener spacing, degree of route curvature and unbalance, and noise and vibration issues. Heavier axle loads require a higher track modulus, as track deflection must be limited to values that will not cause fatigue in the fastener elastomer. In addition, low spring rates in some types of fastener designs will permit the rail to rotate outward under lateral loads due to differential compression of the elastomer layer, causing dynamic gauge widening that may reach undesirable levels. While the AREMA Manual for Railway Engineering currently does not specify maximum vertical rail deflections for direct fixation track, normal practice is to limit the deflection to around ⅛ inch [3 mm] under normal service loads. [4] Some fastener designs require even lower deflections, so the designer is encouraged to contact technical representatives of fastener manufacturers for input on this criterion. For additional information on direct fixation rail fasteners, refer to Chapters 5 and 7 and also to TCRP Report 71, Volume 6: Direct-Fixation Track Design Specifications, Research, and Related Material (TCRP Project D-7, Task 11). Fastener spacing, like the spacing of ties in ballasted track, is a factor in the modulus of direct fixation track; a common spacing for fasteners is 30 inches [762 millimeters]. The fastener stiffness divided by the fastener spacing gives the rail support modulus: a f k = µ where u is the rail support modulus, lb/in/in [kN/mm/mm] kf is the fastener stiffness, lb/in [kN/mm] as confirmed by testing a is the fastener spacing, in [mm] The following example uses a fastener stiffness of 100,000 lb/in (17.51 kN/mm) at a spacing of 30 inches (760 mm): 23,333 30 100,000 a = µ in lb in inlbf k == ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ =×= 204.32 1000 m 760 17.51 a = µ mm N kN N m mmkNf k The dynamic spring rate of most natural, rubber-based, elastomeric, direct fixation rail fasteners is 10% to 100% higher than the static spring rate due to the material relaxation properties of the elastomer. Dynamic spring rate can be most easily visualized by considering that the elastomer

Track Structure Design 4-47 has not fully recovered when the next wheel load is applied. Low ratios of dynamic to static stiffness are achieved with natural rubber fasteners with low shape factor or in shear. The net effect of the dynamic spring rate being higher than the static spring rate is that the rail support modulus under normal train speeds is higher than static fastener tests would indicate. It is important that both static and dynamic fastener spring rate testing be conducted on rail fasteners, because the dynamic stiffness is a more accurate indicator of the fastener’s performance under traffic than the static stiffness. The Dynamic/Static Stiffness Ratio (D/S Ratio) is dependent on the type of elastomer material used as well as the configuration of voids in the elastomer. In general, a lower D/S Ratio is desirable because the resulting lower track support modulus reduces impact loads and vibration forces transmitted to the invert. However, elastomers with higher D/S Ratios may offer greater damping of the dynamic resonance of the rail on the fastener and the so-called pinned-pinned mode of rail bending, which has been implicated as one cause of rail corrugation. 4.4.2.3 Embedded Track The track support modulus for embedded track is very dependent upon the design of the immediate rail support (such as elastomer embedment or elastomer rail boots) and the underlying base slab. For ballasted track that has an overlay of some sort of pavement material (known as “paved track,” as distinct from embedded track), the track modulus will be in the range of ballasted track, 1500–4500 psi [10.3 to 31.1 kN/mm2]. See Article 4.4.2.1 for ballasted track modulus values. If the pavement extends down into the tie crib areas, and, especially if the pavement is constructed underneath the ties, the track structure behaves more like a slab. Ballasted track equations are not valid for the latter case. Many of the embedded track designs constructed in the 1980s and 1990s were essentially direct fixation trackwork installed in open troughs formed in an underlying concrete slab. For such designs, where the trough infill material provides little or no structural support, or where only elastomeric side pieces are used, the track modulus is identical to the direct fixation track analysis indicated in Article 4.4.2.2. Except in very special applications/installations, such track designs are generally no longer used in North America, largely due to the adoption of the relatively inexpensive and hence popular booted rail embedded track design. Details similar to open trough designs are seen in photographs of some European projects, but engineering details are not readily available. Embedded track designs of this sort are generally no longer recommended since the voids necessary for the direct fixation rail fasteners can collect moisture, leading to corrosion that can possibly compromise the structural and electrical integrity of the system. Determining the track modulus for most embedded trackwork designs is more difficult than for direct fixation track for the following reasons: • The rail is continuously supported. The Talbot premise of beam supports on an elastic foundation does not apply. • Rail deflections can be extremely small. • The spring rate for the rail support material is neither known nor easily determined. • The subgrade stiffness, which is not well known, strongly affects the track stiffness.

Track Design Handbook for Light Rail Transit, Second Edition 4-48 Track modulus values have very little meaning for designs where the bare rail is completely encased in concrete without rail boots, such as occurs in some “bathtub” embedded track designs. Rail deflections, if any, are extremely small—possibly as low as 0.001 inches [0.025 millimeters]. The corresponding track modulus is extremely high and largely dependent on the deflection, if any, of the underlying subgrade. Extremely stiff track of this type is highly prone to corrugation and therefore not recommended. An embedded track design with limited resiliency, such as a rail trough filled with a polyurethane/cork mixture, could have track deflection measurements under a 12,000-pound [53,379-N] wheel load in the range of 0.002 to 0.010 inches [0.050 to 0.25 millimeters]. The smaller deflection corresponds to an average force per unit deflection of the rail of approximately 2,000,000 lb/in [350,256N/mm]. The track support modulus is thus very high. A more complex evaluation would be needed for a design that uses rigid, non-resilient, direct fixation rail fastener plate supports. For concrete infill, the track modulus would be extremely large. For an elastomeric or asphalt infill, the track modulus would be calculated from the rail deflection between rigid supports using conventional structural continuous beam formulas. However, the compliance of the base or subgrade would control the track stiffness. The “rail boot” design, first employed in Toronto circa 1990, has become common in embedded track design. The boot provides a continuous elastomeric pad under the rail base, providing resiliency based on voids in the boot configuration, rail perimeter mechanical protection to the surrounding embedment materials, and electrical insulation to isolate the rail and prevent stray current leakage. Representative track moduli for embedded track with rail boot may be estimated from data derived by one manufacturer. The manufacturer’s rail boot design uses a 73 Durometer elastomer with a 5/16-inch [7.9-millimeter] thickness under the rail base that has ribbed shape factors for resiliency. The static track modulus for this design varies, but is in the range of 15,000–30,000 lb/in2 [103–207 N/mm2]. An additional ribbed elastomer layer can be used under the boot, increasing pad thickness to ¾ inch [19 mm] and decreasing track modulus by approximately 50% to 65%.[3] Note that the track modulus change is not a linear function of elastomer thickness, but varies with the elastomer pad shape factor and use of a foamed elastomer. Where the assumption of a linear elastomeric pad deflection is reasonable, a rough estimate of track modulus can be obtained by using a rail deflection of 15% of the elastomer pad thickness.[4] Elastomers that are routinely strained more than about 25% of their thickness begin to creep and attain a permanent set. 4.4.3 Transition Zone Track Modulus Track modulus can vary dramatically among various track types. Well-maintained ballasted track in embankment soil of optimum density, where timber or concrete cross ties are supported by a stipulated depth of ballast and subballast, can have a track modulus as low as 2,500 psi [17.2 N/mm2] or as high as 7,000 psi [48.3 N/mm2]. Concrete cross tie and timber cross tie track with elastic rail fastenings tend toward the higher end of the scale. Embedded or direct fixation track, where a concrete base slab supports the rail, typically have a higher modulus value and greater

Track Structure Design 4-49 stability, as do non-ballasted “open” deck bridge structures where the rail is supported on rigid structural abutments and spans. 4.4.3.1 Interface between Track Types The transition interface points between embedded and ballasted track segments and between direct fixation and ballasted track are typically locations of sudden major changes in track modulus. These differential track modulus values, if substantial (greater than 3,000 psi [20.7 N/mm2] difference between trackforms), generate a weak spot in the overall track structure leading to high maintenance and likely breakdown of the track. If special design consideration is not given to such areas, particularly in line segments where the transit vehicles operate at speeds greater than typical yard operation, the ballasted track will invariably settle and the stiffer adjacent track installation may incur track component and structural damage. Every at-grade railway/roadway crossing also experiences the same track modulus changes. The passengers will experience degraded ride quality as an abrupt transition in the form of vertical acceleration, similar to an automobile hitting a pothole or bump in a highway. The abnormal ride quality is more pervasive when traveling from stiff track (high modulus) to the more flexible track (low modulus) than it is in the other direction. A typical example is the interface between an open deck bridge and adjoining ballasted track. Railroads have long been aware of track alignment problems in these areas and have attempted to compensate by installing transition or approach ties similar to those shown on AREMA Plan basic Number 913. Various arrangements of long-tie installations are used on different railroads, sometimes with an incremental decrease in the cross tie spacing. The objective of these designs is to gradually stiffen the ballasted track structure over an extended distance, thereby reducing the abrupt change in track stiffness at the bridge abutment. Transition tie arrangements have also been placed at the ends of concrete tie installations where the track modulus differential between the concrete and timber cross ties often results in additional surface maintenance requirements. Similar conditions exist in transit track design where installations between ballasted track and both embedded and direct fixation track cannot be avoided. Special transition track design must be considered to maintain an acceptable ride quality at these locations without incurring excessive maintenance costs. TCRP Research Results Digest 79: Design of Track Transitions, which reports results from TCRP Project D-7, Task 15, includes extensive information on track transition areas, both from actual installations and theoretical analysis, along with design guidelines. 4.4.3.2 Transition Zone Track Design Details In North America, the usual design to compensate for the track modulus differential is to use a reinforced concrete transition slab (also commonly called an “approach slab”) to support the ballasted track. These transition slabs (see Figure 4.4.1) extend from the end of the structure abutment or the end of embedded track slab, a minimum of 20 feet [6.1 meters] into the ballasted track section. The top of the slab typically is located 12 inches [300 millimeters] below the bottom of the ties immediately adjacent to the stiffer track, gradually increasing to 14 inches [355.6 millimeters] at the far end of the slab. This design replaces compressible subballast materials with a stiffer base, while also gradually decreasing the thickness and compressibility of the ballast layer.

Track Design Handbook for Light Rail Transit, Second Edition 4-50 Figure 4.4.1 Track transition slab Center-to-center distances between cross ties are generally reduced in the transition slab section to provide additional stability and increase the track modulus. Cross tie lengths are also often increased incrementally for the same reason, and such arrangements have been standard details for most freight and passenger railroads and many transit agencies for a century or longer. Curiously, computer simulations conducted by TCRP Project D-7, Task 15, concluded that such

Track Structure Design 4-51 measures had little benefit in terms of either reducing rail deflection or increasing track stiffness.[6] Additional research might be warranted into this topic. Even a well-designed transition zone will experience some track surface degradation during operation, requiring periodic inspection and resurfacing to avoid pumping track conditions. Drainage conditions and design have a key role in establishing a high-performance transition zone. If the surrounding ballasted roadbed at the transition slab is well drained, the propensity for settlement will be reduced. On one project, with a transition zone that is always dry because it is underneath a building constructed over the trackway, 25 years of operation has resulted in no discernible settlement. 4.4.3.3 Transition Zone Conditions The vertical deflection of the rail with a transition zone resembles a sine curve produced by the wheel load both entering and leaving the stiffer track section. The rails in the ballasted track portion will ultimately show a downward deflection approximately 3 feet [1 meter] from the transition point or end of direct fixation or embedded concrete slab, with a resulting upward force of approximately 3 feet [1 meter] into the direct fixation or embedded track portion. This, by itself, is not an issue if both sides of the transition are ballasted track, as would occur at the abutment of a ballasted track bridge. However, it is a concern where the stiffer side of the interface is either direct fixation or embedded track. The rail sine wave merely disturbs the ballasted track but attacks the direct fixation or embedment track installations with higher vertical loadings, leading to deterioration of components and track conditions. 4.4.3.3.1 Transition from Ballasted Track to Direct Fixation Track The ballasted track side of the transition zone, even with a transition slab, cannot consistently produce a uniformly varying track modulus due to the tendency of ballast to compact, pulverize, and become fouled. Such deterioration leads to settlement voids, hard spots, and pumping track. Regular maintenance of the ballast is needed to protect the track structure’s components and maintain ride quality. Direct fixation fastener design continues to evolve, and a wide range of fastener spring rates is available. A direct fixation track modulus of 3,333 psi [23.1 N/mm2], which compares favorably with conventional concrete cross tie installation, is now possible. Softer direct fixation fasteners in the zone immediately adjacent to the ballasted track transition zone can alleviate some of the transition problems that are not addressed by conventional transition slabs. Gradually increasing the spacing of the transit system’s standard direct fixation fasteners on the approach to the ballasted track limit might also reduce the abrupt change in track stiffness at the interface without adding a special design of direct fixation fastener into the track maintenance inventory. 4.4.3.3.2 Transition from Ballasted Track to Embedded Track Embedded track design continues to evolve and improve; however, the rail deflections that would be required to match typical ballasted track modulus values are difficult to achieve in embedded track. The track sine wave phenomenon in the rail places extremely high bending forces in the rail contained within the embedded track zone immediately adjacent to the ballasted-to- embedded track transition point. The differential in track modulus between ballasted and embedded track may be too large to overcome by introducing a flexible rail support in only the ballasted area adjacent to the interface. Introduction of additional resiliency in the embedded

Track Design Handbook for Light Rail Transit, Second Edition 4-52 track in advance of the interface is suggested. In the case of embedded track using rail boot, this might require placing additional elastomeric pads beneath the boot or transitioning to a trough type of embedded track with additional polyurethane grout beneath the base of rail. Keep in mind that elastomers provide resiliency only if they have some void into which they can bulge. Transition areas that are operated at slow speeds (such as those that occur at the edge of a shop building apron) typically don’t require any special treatments. As speeds increase, more thought should be given to gradually reducing the track stiffness over a time interval of a second or more. This may require measures on both sides of the interface location. 4.4.3.3.3 Design Recommendation The goal of any design to improve the performance of the transition track zone is to minimize dynamic loads by equalizing or smoothing the vertical support condition and the dissipation of dynamic energy across the transition. The track designer must eliminate, reduce, or accommodate the pronounced sine curve reaction in the rail through the transition zone. Eliminating or reducing the sine curve using conventional track components is more easily achieved in direct fixation track than in embedded track. A recommended reading on transition zones is TCRP Research Results Digest 79. It reviews and analyzes various track transitions and designs among ballasted and non-ballasted track forms and structures and offers guidance to improve track and operating performance. TCRP Research Results Digest 79 says the following transition designs can be considered the most efficient for rail transit applications, based on a literature review and GEOTRACK analysis: • Matching the vertical fastener stiffness of direct, ballasted deck, or open deck bridges to the track modulus and rail deflection behavior of the at-grade ballasted track, without modifications of the at-grade track, provides the most efficient and cost-effective design. Direct fixation fasteners with stiffness values between 100 and 200 kip/inch [17,500 and 35,000 N/mm] deflection, are compatible with ballasted tracks with average stiffness subgrades (Er values between 5 and 15 ksi [34.5 and 103.4 megapascals]). The analysis showed the rail deflection differentials for these designs to be less than 0.04 inches [1 millimeter] for wheel loads of 12, 15, and 22.5 kips [5,443, 6,804, and 10,206 kg force respectively]. • The use of a rubber pad, bonded to the bottom of the concrete ties on ballasted deck bridges, provides adequate resilience to transition to ballasted track on an average stiffness subgrade. Modeling suggests that the rubber pad stiffness should be 100,000 lb/in or higher. • Low stiffness subgrades with Er values less than 5 ksi [34.5 megapascals] require some modification in addition to the controlled resilience of the structure track. These subgrades are typically made up of cohesive soils (clays and silts) with moisture contents higher than optimum. Increasing the modulus of track on a low stiffness subgrade requires modification of the physical state of the soil and/or installation of a structural reinforced layer between the ballast and subgrade, such as a concrete approach slab. • Avoid the creation of weak subgrade conditions during new construction by careful soils selection and the application of geotechnical best practices.

Track Structure Design 4-53 Additional suggested design features include the following: • Diverting surface runoff from the direct fixation track or embedded track sections so that it doesn’t enter the transition area. In direct fixation track, provide an end barrier wall and drain surface runoff to the side of the track beyond the embankment. In embedded track, provide a surface drain within 5 feet [1.5 meters] of the end face of the embedded track. • Using a series of progressively longer concrete ties leading up to the abutment or embedment face of the non-ballasted track. Additional abutment width should be provided to accommodate a wider concrete base track slab and a wider embankment section to retain the widened track structure. • Providing lateral perforated track drains at the ends of the base slabs to carry off base slab runoff. • In embedded track, encasing the last 2 feet [60 cm] of booted rail prior to the beginning of the ballasted track in 60 durometer polyurethane. This will provide a track stiffness transition and protect the rail and pavement against damage that could occur when mechanically raising and tamping the adjoining ballasted track. The use of porous filler materials, such as cork of shredded rubber, can enhance the resiliency. 4.5 BALLASTED TRACK Ballasted track is the most prevalent track type used in light rail transit. While ballasted track for light rail transit resembles conventional railroad track in appearance, its design may have to contend with issues such as electrical isolation and acoustic attenuation. In addition, ballasted LRT track may include continuous welded rail on an alignment that includes curves far sharper and grades far steeper than would ever be encountered on a freight railroad or even a “heavy rail” transit route. Proper design of the roadbed, ballast, and subballast elements of the track structure is a key issue. It is essential in providing an adequate foundation for the track so as to minimize future maintenance requirements. Roadbed and ballast sections should be designed to minimize the overall width of the right-of-way while providing a uniform and well-drained ballast foundation for the track structure. 4.5.1 Ballasted Track Defined Ballasted track can be described as a track structure consisting of rail, tie plates or fastenings, cross ties, and the ballast/subballast bed supported on a prepared subgrade. The subgrade may be a compacted embankment or fill section, an excavation or cut section, a bridge structure, or a subway tunnel invert. Ballasted track is generally the standard for light rail transit routes that are constructed on an exclusive right-of-way. Ballasted track can be constructed to various designs, depending on the specific requirements of the transit system. Depending on the portion of the system under design and presuming for the moment that stray traction power currents are not an issue, a satisfactory ballasted track design

Track Design Handbook for Light Rail Transit, Second Edition 4-54 could consist of either timber cross ties with conventional tie plates, cut spikes, and rail anchors or concrete cross ties with elastic rail fastenings that incorporate conventional insulating components (so as to retain traction power currents within the rail). While the loadings typically are limited to those of the light rail vehicles only, heavier loading standards may be required. The track designer must consider that the heaviest loading may be generated by the maintenance-of- way equipment. In addition, ballasted track may need to accommodate freight railroad loadings where the track is to be shared with a commercial railroad. Light rail track structural loading is one-quarter to one-third of that imposed on freight railroad tracks. (Light rail bridges and aerial structures must also take these design parameters into consideration. Refer to Chapter 7 for structural design details.) 4.5.2 Ballasted Track Criteria To develop ballasted track design, the following track components and standards must be specified: • Rail section. • Track gauge. • Guarding of curved track and restraining rail features. • Rail fastenings and tie plates. • Type of track cross ties and corresponding track structure to suit operations. 4.5.2.1 Ballasted Track Rail Section and Track Gauge Refer to Article 4.2 and Chapter 5 of this Handbook for guidance on determining rail section, track gauge, and flangeway requirements. 4.5.2.2 Ballasted Track with Restraining Rail Refer to Article 4.3 herein for determining requirements, locations, and limits for guarding track with restraining rail. Specific details for various types of restraining rail designs are included in Chapter 5. 4.5.2.3 Ballasted Track Fastening Refer to Chapter 5 for requirements concerning cross tie rail fastenings. A key issue for rail fastenings on ballasted track cross ties for transit use is providing sufficient electrical isolation to deter the migration of stray traction power currents. 4.5.3 Ballasted Track Structure Types There are generally two standard designs for track structures on ballasted track: • Timber cross tie track. • Concrete cross tie track. Both plastic and steel cross ties have been used in railway track construction, but they have not gained wide acceptance. See Chapter 5 for additional discussion on alternative cross tie materials. Many transit systems have used both timber and concrete cross ties. Up until about 2000, the main line tracks on most new LRT installations were usually constructed using concrete cross ties

Track Structure Design 4-55 with standard rail insulation. The yard maintenance facility tracks were generally built with timber cross ties either with or without insulated fasteners. The non-insulated construction was appreciably cheaper to construct. Special trackwork in both main track and yard track was commonly constructed on timber switch ties, largely because concrete switch tie designs had not matured and were hence extremely expensive. With very few exceptions, projects since about 2000 have mostly used concrete cross ties throughout, including yard tracks and special trackwork. This is largely because the cost of concrete ties in relation to high-quality timber ties with insulated rail fastenings is now comparable. Improved designs also show more promise for actually fulfilling the 50-year service life long claimed for concrete ties. By contrast, LRT systems constructed with timber typically face a need to replace a huge percentage of their cross ties during a fairly brief period—about 20 to 30 years after original construction. Also, transit yard design has been trending toward full electrical isolation of yard tracks from ground, separate traction power substations notwithstanding. Whether this is fully justified is an open question. Ballasted track design can result in a suitable track structure using either timber or concrete cross ties. The differential track support or track modulus dictates the quality of the track, the ride, and future maintenance requirements. Concrete cross tie ballasted track provides a more reliable track gauge system and tighter gauge construction tolerances. The higher track modulus results in a smoother ride with less differential track settlement. Chapter 2 documents the types and magnitudes of loads transferred from the vehicle wheel to the rail. The rail must support the vehicle and the resulting loads by absorbing some of the impact and shock and transferring some forces back into the vehicle via the wheels. The initial impact absorber on the vehicle is the elastomer in the resilient wheels (if used) followed by the primary suspension springs and then the secondary suspension system. The initial impact absorber on the track is the rail, particularly the rail head, followed by the fastening or supporting system at the rail base, and then the remaining track structure. A resilient rail seat pad is used to absorb some of the force on concrete cross ties. On timber cross ties, the resiliency in the wood itself acts as the absorber. All components absorb and distribute a portion of the load. The track structure’s design (degree of resiliency) dictates the amount of load distributed to the rail and track structure and the magnitude of force returned to the wheels and vehicle. 4.5.3.1 Ballasted Track Resilience Ballasted track design allows partially controlled rail deflection in both the vertical and horizontal directions. This phenomenon of rail action contributes to successful track operation by distributing the load to the surrounding track components and structure. Specific track design decisions must be made regarding the type of track structure (timber cross tie/concrete cross tie) and corresponding track structure resiliency or track support stiffness. Rail supported on timber cross ties and a moderate ballast/subballast section using conventional rail fastenings consisting of tie plates, cut spikes, and rail anchors results in a track modulus range of 2000 to 2500 lb/inch per inch of rail [14 to 17 N/mm2].

Track Design Handbook for Light Rail Transit, Second Edition 4-56 Resilient rail base pads are placed on concrete cross ties to protect the concrete tie seat and to impede the impact and vibration associated with wheel passage from migrating from the rail to the cross tie. Resilient rail base pads are a determining parameter of track modulus. A reduced pad height of 1/4 inch [6 millimeters] and a very stiff elastomer or polyethylene pad produce a stiff track support resulting in an increased rail support modulus. Rail supported on concrete cross ties and an ample ballast/subballast section has a track modulus range of 4,500 to 6,500 lb/inch per inch of rail [31 to 45 N/mm2]. 4.5.3.2 Timber Cross Tie Ballasted Track On many light rail transit systems, particularly legacy systems and systems constructed in the early 1980s, timber cross ties were considered to provide sufficient electrical isolation. Specific measures to insulate the track were not used because other measures were either taken or already in place (such as utility bonding and drain cables) to address traction power stray current. Typically, non-insulated rail fastenings were employed only in yard tracks, where the yard has its own traction power substation and stray currents are unlikely to leave the immediate site. Non- insulated, ballasted track was also occasionally used in rights-of-way where there were no parallel utilities; however, the occurrence of rights-of-way without parallel utilities is an extremely unlikely circumstance and the practice of using non-insulated track in such a situation ignores the fact that stray currents can take very circuitous paths quite distant from the track. Non-insulated track is therefore not recommended, and contemporary designs typically incorporate insulation systems within the cross tie rail fastening to control stray currents close to their source. Timber cross tie ballasted track consists of the rail placed on a tie plate or rail fastening system that is positioned on the cross tie, which is supported by a ballast and subballast trackbed. Timber cross tie ballasted track is generally similar to the concrete cross tie track shown in Figures 4.5.1 and 4.5.2. Figure 4.5.1 Ballasted single track, tangent track (concrete cross ties) 4.5.3.2.1 Timber Cross Tie Rail Fastenings Conventional tie plates, cut spikes, and rail anchors were considered sufficient for ballasted track installations using timber cross ties for railroad and legacy rail transit track. However, current transit track design generally includes insulation in the rail fastening system so as to protect the negative return rail from stray electrical currents.

Track Structure Design 4-57 Figure 4.5.2 Ballasted single guarded curve track (concrete cross ties) Although wood is an insulating material, timber cross ties provide only a limited barrier against stray current and become less effective in that regard over time. Therefore, timber cross ties generally utilize rail fastenings that are insulated at the base of the tie plate or fastening plate. A typical detail places a high-density polyethylene (HDPE) pad, -inch [9-millimeters] thick, between the timber cross tie and the tie plate. The HDPE pad will project a minimum of ½ inch [12 millimeters] beyond all sides of the steel fastening plate so as to minimize the chance of the edges being bridged by conductive debris. A special insulating collar/thimble is positioned in the anchor screw spike hole to isolate the screw spike from the steel fastening plate. The screw spikes are sometimes epoxy coated for additional electrical isolation. Alternatively, the hole drilled in the cross tie can be partially filled with a coat tar epoxy or other insulating gel prior to installing the spike, thereby forcing the insulating material into as many crevices and voids as possible. For additional design information on timber cross tie fastenings, refer to Chapter 5. 4.5.3.2.2 Timber Cross Ties Timber cross ties have been standard for light rail transit installations for years and continue to be the standard for older, established transit agencies. Life cycle cost comparison of timber ties and concrete ties must be performed using a uniform baseline, including all fastenings and hardware needed for each type of tie. The tie spacing for timber ties is generally shorter than for concrete ties, which results in not only more cross ties, but also less ballast per unit of track length. These considerations must be factored into the analysis. Conventional rail anchors projecting into the ballast section will create a stray current leakage path, particularly in areas where the ballast is wet and/or contaminated, which is another issue to be considered. Also, the material cost for timber cross ties can vary widely over a short period of time. That said, many transit agencies still continue to use timber ties with satisfactory results. Broad gauge LRT systems (all of which are legacy operations dating back to the 19th century) generally select timber cross ties. It is unclear whether the deciding issue is first cost of special design concrete ties or a disinterest in change. Timber cross ties (if selected) for a transit system should be hardwood (e.g., oak, maple, or birch), generally with a cross section of 7 by 9 inches [175 by 230 millimeters]. In the western

Track Design Handbook for Light Rail Transit, Second Edition 4-58 portions of North America, Douglas fir is readily available and considered equivalent to eastern hardwoods. For additional information on timber cross ties, refer to Chapter 5. Determining timber cross tie spacing for transit track is discussed in Article 4.5.4. 4.5.3.3 Concrete Cross Tie Ballasted Track Concrete cross ties have become nearly universal for new light rail transit installations. They have been shown to have a longer service life, have lower life cycle costs, provide a higher track modulus (which equates to better ride quality), and incur lower track surfacing maintenance costs. When the cost of procuring and installing insulated rail fastenings on high-quality timber cross ties is considered, concrete cross ties have a very favorable first cost, particularly considering that they can generally be spaced more widely than timber ties. The only exceptions in recent times have been extensions or rehabilitation projects on existing systems that have traditionally used timber cross ties. In some instances, those systems also use broad track gauge, which may have tipped first cost economics in favor of insulated timber versus concrete cross ties. The concrete cross tie is typically insulated at the base of the running rail, thereby protecting the base of the rail from potential stray current leakage. Concrete cross tie ballasted track consists of the rail placed in the rail seat area and the tie supported by a ballast and subballast trackbed, as shown in Figures 4.5.3 and 4.5.4. 4.5.3.3.1 Concrete Cross Tie Rail Fastenings Experimental concrete cross tie designs first appeared around 1920, but they were generally unsuccessful, largely due to failures in the rail fastening systems. The current success of the concrete cross tie is partly due to the introduction of elastic (spring) clip fastenings at the rail hold down location, which replace the spikes and threaded fasteners used in early designs. Fastening designs have also evolved to meet new requirements for electrical isolation. The insulating barrier must be at the base of the rail or mounting surface to provide electrical isolation of the rail from the surrounding track components. The insulating barrier consists of a base rail pad and clip insulators for the edges of the rail base. As shown in Chapter 5, Figure 5.4.1 of this Handbook, the rail is fully insulated from the mounting surface. Figure 4.5.3 Ballasted double tangent track (concrete cross ties)

Track Structure Design 4-59 Figure 4.5.4 Ballasted double curved track (concrete cross ties) The concrete cross tie design includes the specific type of elastic fastening system (e.g., spring clip) with insulating rail seat pad and rail base clip insulators. The two elastic clips at each rail seat provide sufficient toe load to the rail base to act as the longitudinal rail anchor, eliminating the conventional rail anchors used with timber cross ties. 4.5.3.3.2 Concrete Cross Ties The typical transit concrete cross tie is made of prestressed, precast concrete produced in a factory with climate controls for the curing process. The ties are generally 10 inches [255 millimeters] wide and 8’ 3” [2515 millimeters] long, measured at the base of tie. So as to facilitate removal from the molds, the tie is vertically tapered, with slightly smaller plan dimensions at the top of the tie. Tie thickness is generally 7 ½ inches [190 millimeters] at the rail seat and 6 ½ inches [165 millimeters] at the center of the tie. For additional information on concrete cross ties refer to Chapter 5. 4.5.4 Cross Tie Spacing The optimal spacing of cross ties in ballasted track is dependent on two issues: Vertical support, so as to distribute the wheel loads through the ballast and subballast such that the underlying soils are not overstressed. Lateral support, so that the track is adequately restrained against lateral movement due to thermal stresses and loadings in the rails. 4.5.4.1 Cross Tie Spacing—Vertical Support Considerations Ballasted track structure design is dependent on the vehicle wheel load, a predetermined track modulus target or standard, the selected rail section, the type and size of tie, and the depths of ballast and subballast. These are combined to meet the criteria established by AREMA for both ballast pressure and subgrade pressure. Ballasted track designs can meet or exceed the AREMA pressure requirements by altering the variable parameters (track modulus, tie spacing, and ballast depth) as needed. As a guideline, the following sample calculations—based on the formulae from Talbot[5], Timoshenko and

Track Design Handbook for Light Rail Transit, Second Edition 4-60 Langer[11], and Hay[2]—are provided for design of ballasted track with timber or concrete cross ties assuming the following typical LRT installation parameters: Rail Section 115 RE Vehicle Load per Wheel 12,000 pounds [5,400 kilograms] Track Modulus Timber Tie 2,500 lb/inch per inch [17.2 N/mm2] Concrete Tie 5,000 lb/inch per inch [34.5 N/mm2] Desired Load Transfer to Ballast <65 psi [0.45 MPa] Subgrade <20 psi [0.14 MPa] Ballast Depth 10 inches [255 millimeters] Subballast Depth 8 inches [200 millimeters] Tie Sizes Timber 7 x 9 x 102 inches [180 x 230 x 2590 millimeters] Concrete 7.5 x 10 x 99 inches [190 x 250 x 2515 millimeters ] Design Calculations: Tie Seat Load = β × a × P (Timoshenko and Langer[11]) where a = tie spacing (variable) P = axle load = 107 kN (24 kips) 1/4 4EI u = ⎟⎠ ⎞⎜⎝ ⎛β Timber Tie: u = track modulus = 2,500 lb/inch per inch [17.2 N/mm2] Concrete Tie: u = track modulus = 5,000 lb/inch per inch [34.5 N/mm2] E = modulus of rail steel = 30 x 106 psi [206,800 N/mm2] I = moment of inertia of 115 RE rail = 65.9 in4 [27.4 x 106 mm4] Tie Bearing Area = tie width x tie length Timber = 9 inches x 102 inches [230 x 2590] = 918 sq inches [595700 mm2] Concrete = 10 inches x 99 inches [250 x 2515] = 990 sq inches [628750 mm2] [5] [2] Talbot Depth Ballast Width Tie Area Bearing Tie Load Seat 1.23 =Centerline Tie at Load Subballast Hay Area Bearing Tie32 Load Seat Tie =Load Ballast ×⎟⎠ ⎞⎜⎝ ⎛

Track Structure Design 4-61 Subgrade Load at Tie Centerline is similar to subballast load calculation except depth includes ballast and subballast heights. Using the above formulas, Table 4.5.1 presents the values according to the parameters. Tie spacing can be determined from this table. Neither the AREMA recommended maximum ballast pressure, 65 psi [0.45 MPa], nor the maximum subgrade pressure, 20 psi [0.14 MPa], should be exceeded. Table 4.5.1 Ballasted track design parameters Tie-Ballast Load Subballast Load Subgrade Load Ballast + Subballast Cross Tie Tie Seat Load 9” [230] Tie 10” [250] Tie 10” [255] Ballast Depth 18” [455] Track Modulus Spacing inches [mm] Kips [kN] psi [MPa] psi [MPa] psi [MPa] [psi] MPa 2500 lb/in/in [17.2 N/mm2] 20” [510] 11.4 [50.7] 18.5 0.127 n.a. n.a. 13.7 0.094 7.6 0.096 β = 0.0237/in [0.00093/mm] 24” [610] 13.6 [60.7] 22.1 0.152 n.a. n.a. 16.4 0.113 9.1 0.115 27” [685] 15.3 [68.2] 24.9 0.171 n.a. n.a. 18.5 0.127 10.3 0.130 30" [760] 17.0 [75.6] 27.6 0.189 n.a. n.a. 20.5 0.141 11.4 0.144 32” [810] 18.1 [80.6] 29.4 0.202 n.a. n.a. 21.8 0.150 12.1 0.153 5000 lb/in/in [34.5 N/mm2] 20” [510] 13.5 [60.0] n.a. n.a. 20.4 0.142 16.8 0.115 9.3 0.115 β = 0.0282/in [0.0011 /mm] 24” [610] 16.1 [71.8] n.a. n.a. 24.3 0.170 20.0 0.138 11.1 0.138 27” [685] 18.1 [80.6] n.a. n.a. 27.3 0.191 22.5 0.155 12.5 0.155 30” [760] 20.1 [89.5] n.a. n.a. 30.3 0.212 25.0 0.172 13.9 0.172 32” [810] 21.4 [95.3] n.a. n.a. 32.3 0.226 26.6 0.183 14.8 0.183 Note: 1 MPa = 1 N/mm2 The preceding computations are representative of the calculations needed to design the ballasted track structure. The parameters that alter the actual design are predetermined track modulus, type of tie (timber or concrete), depth of ballast and subballast, and tie spacing. The challenge for the track designer is to combine these parameters to achieve the best life cycle costs and lowest maintenance costs. 4.5.4.2 Cross Tie Spacing—Lateral Stability Considerations The above calculations determine the cross tie spacing and affect the track modulus or the vertical track stiffness. Lateral track stability can also affect cross tie spacing. For the curve radii typically encountered in railroad work, if there are sufficient cross ties to provide vertical support, lateral restraint is rarely an issue. This is not true in LRT track design. The horizontal track alignment for a light rail transit system can include curves far more severe than curves on railway systems such as metro rapid transit, commuter rail, or freight railroads. Ballasted track alignment is far more difficult to construct and maintain in tight radius curves. Special consideration should therefore be given to increasing lateral track stability by reducing the cross tie spacing.

Track Design Handbook for Light Rail Transit, Second Edition 4-62 Lateral track stability is provided by ballast friction contact along the sides and bottom of the tie and by the end area of the tie. The end area of the tie provides a calculated degree of lateral stability; however, increasing the ballast shoulder width beyond an 18-inch [450-millimeter] limit provides no increase in stability. Reducing cross tie spacing, thereby increasing the number of ties, can increase lateral track stability. Timber cross ties have been proven to provide greater lateral stability than concrete ties, generally because the ballast’s sharp edges penetrate the tie surfaces, increasing the friction and locking the cross tie in position. On the other hand, the concrete tie’s increased weight also provides increased lateral stability. To improve the lateral stability of concrete cross ties, some tie manufacturers have developed a serrated or “scalloped” side tie surface, increasing the ballast’s locking capabilities. As a guideline, the track designer should consider reducing the conventional cross tie spacing calculated in the previous article by 3 inches [75 millimeters] for curves with radii less than 1000 feet [300 meters] and an additional 3 inches for curves tighter than 500 feet [150 meters]. To improve lateral stability, especially with conventional smooth-sided concrete ties, a tie anchor can be bolted to the tie. The tie anchor is a vertical blade penetrating below the base of the tie into the ballast bed. Tie anchors can be attached to all or alternate ties in the curve. Installation of tie anchors is a manual process that disturbs the ballast consolidation, requiring the track to be retamped. These devices appreciably complicate track construction and should be considered only as a last resort. 4.5.5 Special Trackwork Switch Ties Concrete switch ties have been developed by the railroad industry to reduce installation costs and long-term maintenance on heavy haul freight lines. Concrete switch ties are initially expensive to design and fabricate, but now that some standard designs exist, procurement costs are coming down compared to pricing during the 1990s. As of this writing (2010), most transit agencies still use standard timber hardwood ties for special trackwork for both main line and maintenance facility and storage yard installations. Concrete switch ties are becoming more popular, but have not become universally accepted, even on projects that otherwise use concrete cross ties. The situation is evolving, and it seems reasonably certain that concrete will become the switch tie material of choice for most LRT projects in the near term. Turnout standards vary among transit agencies. Therefore, various concrete tie geometric layouts and designs would be required to meet the requirements of each agency. Standardization and simplicity in turnout tie design is advancing and is required to allow the transit industry to develop a uniform, economical, standard concrete switch tie set for various turnout sizes. 4.5.5.1 Timber Switch Ties The present standard for timber switch ties is domestic hardwoods. In the eastern United States, oak is the preferred species for switch ties. In the western United States, Douglas fir is the predominant species for both switch ties and cross ties.

Track Structure Design 4-63 Tropical hardwood ties, manufactured from species such as Bonzai, Ekki, and Azobe have been used in North American railway and transit trackage with mixed success. The reader is cautioned about using these tropical woods. Thorough research on the specific wood of interest and the origin of the wood is recommended before a procurement is undertaken. Use of the correct botanical names is critical. Several species of Azobe wood exist, each with significantly different characteristics, and the inferior species are subject to rapid decay. Obtaining the correct material cannot be guaranteed without continuous on-site inspection at the saw mill as the species are nearly impossible to differentiate after the bark has been removed. See Chapter 5 for additional information on tropical hardwoods. In North America, the typical timber switch tie is generally a 7 x 9 inch [180 x 230 millimeter] section in various lengths ranging from 9 to 17 feet [2,750 to 5,182 millimeters]. Overseas, timber switch ties are typically much thinner and somewhat wider. Such ties are not recommended for North American use. Extra long timber switch ties, 22 feet [6,710 millimeters] and longer, may be required to accommodate special trackwork locations, such as crossovers and double crossovers where the track centers remain at a standard width. Alternate long-tie designs exist where two shorter ties are abutted and spliced together by a hinged connection. This design allows one track to be removed or worked on while the other track remains in service. The abutted tie connections can sometimes alternate in location (within the track gauge areas) between the two tracks, improving the installation’s stability. The same articulated configuration can also be used with concrete crossover ties. Similar to main line timber cross ties, timber switch ties may require an insulated switch plate design to protect against stray current leakage. Generally, insulation details for switch and frog plates are similar to those used on main line timber cross ties. The dual concern for both stray current control and vibration isolation has occasionally resulted in the installation of special trackwork direct fixation fasteners on timber switch ties. 4.5.5.2 Concrete Switch Ties Concrete switch ties for virtually all size turnouts are now available for a price. However, the design details are generally not published information and readily available as they are proprietary to each manufacturer. Concrete switch tie designs and layouts are different from the timber switch tie arrangements. Tie spacings are increased as the width of the concrete switch ties, approximately 10 inches [250 millimeters], distributes loads over a wider ballast surface area than timber switch ties. For simplicity and because patterns are readily available, spacings of ties beneath switches and frogs often follow railroad practice. The lengths of the concrete switch ties conform to the needs of the special trackwork layout. Unlike timber switch ties, which usually are supplied in length increments of 1 foot [30 cm], concrete switch ties are often supplied in specific lengths for each tie location. The switch tie design includes embedments for mounting rails and fastenings for special trackwork plates. Both embedded shoulders and single rail fastener plates have been used outside of the areas of switch and frog plates.

Track Design Handbook for Light Rail Transit, Second Edition 4-64 Similar to timber switch tie installations, insulated special trackwork plates may be required to control stray current on concrete switch ties. Insulated switch, frog, and guard rail fastening plates may be similar to conventional timber cross tie installations. Standard concrete tie insulated rail fastenings can be used where only individual rails are installed on the switch ties. Generally the rail in special trackwork is installed without any rail cant. For more information on special trackwork timber and concrete switch ties, refer to Chapter 5 of this Handbook. 4.5.6 Ballast and Subballast Ballast is an integral material in the support of the track structure. The quality of the ballast material has a direct relationship to the overall performance of the track structure. The quality, size, and type of ballast material used can improve the performance of the track substructure by providing increased strength to the track system. Concrete cross tie installations normally require a higher quality ballast, a larger gradation of ballast, and a more restrictive selection of rock aggregate. For additional information on ballast material, refer to Chapter 5. 4.5.6.1 Ballast Depth The variables to be considered in establishing the track structure section are discussed above and listed in Table 4.5.1. Additional variables include the track gauge, depth of tie, and superelevation of track curves. Figures 4.5.1 and 4.5.2 illustrate and quantify the general desired design section for ballasted single track. For tangent single track, the minimum depth of ballast is generally measured from the underside of the tie to the top of subballast at the centerline of each rail. For curved superelevated track, the depth of ballast is measured below the low rail in the curve with respect for the top of subballast, as shown in Figure 4.5.2. On tangent multiple track installations, the minimum ballast depth is measured under the rail nearest to the crown of the subballast section, as shown in Figure 4.5.3. On curved multiple track installations, minimum ballast depth is usually measured on each track under the inside rail closest to the radius point, as shown in Figure 4.5.4. Special consideration may be required when the slope of the subballast is in the same orientation as the track superelevation. 4.5.6.2 Ballast Width The width of ballast section is determined by the rail installation and tie length. The ballast shoulder assists in resisting lateral track movement and restrains the track from buckling when the rail is in compression. Continuous welded rail generally requires a ballast shoulder that is a minimum of 12 inches [300 millimeters] wide measured from the end of the tie to the top of ballast shoulder slope. The top slope of the ballast shoulder should be parallel to the top of the tie. The side slope of the ballast shoulder should have a minimum slope of 2 horizontal to 1 vertical. As mentioned in Article 4.5.4.2, the ballast shoulder may be increased in sharp radius curved track to

Track Structure Design 4-65 provide additional lateral stability. The subballast and subgrade sections must be increased to provide sufficient support width if the ballast shoulders are increased. 4.5.6.3 Subballast Depth and Width Subballast is the lower or base portion of the ballast bed located between the base of the ballast section and the top of the roadbed subgrade. Subballast is generally a pit run material with smaller, well-graded, crushed stone. The subballast acts as a barrier separating the ballast section from the embankment roadbed materials and provides both separation and support for the ballast. The subballast layer also acts as a drainage layer allowing water to flow to the embankment shoulders. The ideal subballast material would be nearly impervious so that storm water runoff is quickly shed to the drainage ditches at the sides of the track section and does not have the opportunity to penetrate and soften the subgrade. Many investigations have been made into asphalt underlayment for railroad tracks in lieu of conventional subballast as a method of mitigating weaker subgrades. If the asphalt is sufficiently dense and impervious, it can also add some degree of electrical isolation. The depth of the subballast below the ballast can be determined using the calculations in Article 4.5.4.1. The ballast and subballast are integral parts of the track structure. Track design considers the thickness of both in the calculations to meet AREMA recommendations of 20 psi [0.14 MPa] uniform pressure transmitted to the subgrade surface. The width of the subballast section is determined by the width of the roadbed embankment subgrade. The subballast should extend the full width of the embankment, capping the top surface. To allow for an eventual sloughing of the ballast slope and also to provide a relatively flat area for walking by track inspectors and performance of track maintenance activities, the top of the subballast section should project beyond the toe of the ballast slope a minimum of 24 inches [60 cm]. Since the end slope of the subballast generally conforms to the slope of the underlying embankment, this means that the top of the subgrade must be appreciably wider than the top of the subballast layer. This requirement should be carefully detailed on the typical section drawings. On at least one LRT project, the subgrade was erroneously constructed to the width shown for the top of the subballast layer. As a result, the entire roadbed was too narrow by several feet [about a meter], a condition that was not detected until well into the track construction process, long after the earthwork construction had been thought to be complete. To support embankment materials under special trackwork installations and at-grade road crossings, a geotextile (filter fabric) may be used at selected locations. The track designer should review supplier information on geotextiles and, working jointly with the project’s geotechnical engineers, consider the application of geotextiles weighing about 16 ounce/yd2 [0.5 kilogram/m2]. Double layers might be considered under special trackwork locations. Geogrid and geoweb materials may also be used to stabilize and strengthen the subgrade materials below turnouts and at-grade crossings. These materials augment but do not replace the function of subballast.

Track Design Handbook for Light Rail Transit, Second Edition 4-66 4.5.6.4 Subgrade The subgrade is the finished embankment surface of the roadbed below the subballast that supports the loads transmitted through the rails, ties, ballast, and subballast. The designer should review the geotechnical engineer’s analysis of the subgrade materials/soils along the entire route to determine whether all locations have both uniform stability and the strength to carry the expected track loadings. The geotechnical engineer should be intimately familiar with local soils, particularly if the subgrade soils consist of clays with a high plasticity index. Also, soils are unlikely to be completely uniform over the entire length of the route, and different subgrade preparation treatments may be appropriate at any given location along the project right-of-way. AREMA recommends that, for most soils, pressure on subgrade be lower than 20 psi [0.14 MPa] to maintain subgrade integrity. Uniformity is important because differential settlement, rather than total settlement, leads to unsatisfactory track alignment. The use of geotextiles or geogrids between the subgrade and subballast can be advantageous under some conditions. 4.5.7 Ballasted Track Drainage Drainage of the roadbed in embankment or excavated sections is of utmost importance for a sound track structure. The success of any ballasted track design depends directly on the ability of the trackbed to drain well and the proper maintenance of the overall drainage system. These elements include the rapid runoff of storm water from the ballast across the subballast surface and into a properly designed parallel drainage system so as to carry the runoff away from the track. The parallel drainage system can consist of open ditches, underdrains, and the piping necessary to carry water off the right-of-way, all in accordance with storm water management requirements. Ballasted track, by the nature of its design and exposure, is susceptible to contamination from both railway traffic and the surrounding environment. The ballast stone must be kept clean, and voids must be kept open so that storm water runoff can quickly drain down the subballast layer and into drainage ditches or underdrains. Dirt, debris, and fines that are either dropped or blown onto the trackway will “foul” the ballast section. This contamination creates non-porous or slow- draining ballast shoulders and ballast bed, which can lead to a permanently saturated subgrade. The end result can be deterioration and breakdown of the track structure with “pumping” track. If the ballast is not clean when delivered from the quarry, the contaminants, including stone dust, can impair proper ballast drainage from the very beginning. Many conventional methods are practiced to maintain and restore a free-draining ballasted track structure. These include both ballast shoulder cleaning and complete track undercutting, both with the goal of keeping the ballast clean and free draining. In yard track areas, it is sometimes proposed to structure the storm water detention system by holding water in the voids of the ballast section and gradually draining it into underdrains. Unless the subgrade is extraordinarily firm, this method is not recommended since the saturated subballast and subgrade would deteriorate and track surface would suffer.

Track Structure Design 4-67 4.5.8 Retained Ballasted Guideway Right-of-way constraints and other situations often make it impossible to construct a ballasted track with open drainage ditches alongside of the roadbed. In such cases, a ballasted guideway can be constructed between curbs or walls and drainage provided by an underdrain system. Figure 4.5.5 illustrates a typical curbed ballasted guideway design. The design must carefully consider locations where the underdrain system will outlet and how it can be maintained. Such constrained sections need to be carefully detailed to clearly show the relationships of the track, curbs, underdrains, and other civil/drainage facilities to systems infrastructure elements including OCS poles, underground duct banks, and surface cable troughs. Similar details are used at approaches to elevated structure abutments and at underpasses. Constrained sections at bridge abutments and between undergrade retaining walls can be particularly problematic, particularly in the case of mechanically stabilized earth (MSE) retaining walls, where the MSE walls’ reinforcing strips can effectively make much of the retained embankment “off-limits” for any other structures, particularly anything that might need to be installed by trenching. Any conflicts with underground duct banks for LRT electrical systems must be identified and mitigated during design. Figure 4.5.5 Ballasted track—curbed section Curbed ballasted trackways and similar configurations can also make it extremely difficult and costly for maintenance forces to change out defective cross ties, particularly in areas where the zone between the tracks is occupied by system elements such as catenary pole foundations, duct bank manholes, and hand holes and surface cable troughs. 4.5.9 Stray Current Protection Requirements Because the rails are used for traction power negative return, the track structure design must include an electrical barrier to insulate the rail. Ballasted track generally provides this electrical barrier at the rail fastenings. An insulating resilient material with a specified bulk resistivity

Track Design Handbook for Light Rail Transit, Second Edition 4-68 provides the barrier at the base of the fastening plate on timber cross ties. Concrete cross ties provide the isolation at the base of the rail using a pad on the rail seat and insulating pads between the base of rail and the rail clips. Stray current corrosion protection is a subject described more fully in Chapter 8 of this Handbook. For more information on electrical barriers at rail fastenings, refer to Chapter 5. 4.5.10 Ballasted Special Trackwork The ballasted special trackwork portion of any transit system will require turnout, crossover, double crossover, and crossing diamond designs to match the light rail vehicle’s characteristics, the track spacing configurations, and the real property available for the maintenance facility and LRV storage yard. A common form of ballasted special trackwork in contemporary light rail transit systems consists of four turnouts, two right-hand and two left-hand, paired to act as two single crossovers for alternate main line track operations. Occasionally, operating requirements and/or alignment restrictions may dictate the installation of a double, or “scissors,” crossover consisting of four turnouts and a crossing diamond. Turnouts are used at the ends of transitions from double track to single-track installations as well as at junction points to alternate transit routes and accesses to sidings. Turnouts in the maintenance facility and storage yard areas are generally positioned to develop a “ladder track” arrangement that provides access to a group of parallel tracks with specific track centers. For additional information on ballasted special trackwork design, refer to Chapter 6. 4.5.11 Noise and Vibration The vehicle traveling over ballasted track produces noise and vibration. The impact of this noise and vibration may become a significant annoyance for alignments through otherwise quiet and sensitive areas, such as neighborhoods with schools, churches, theatres, recording studios, laboratories, and hospitals. Ballasted track design has a significant effect on both noise and vibration, with wheel/rail squeal as a prime contributor. However, to be effective, the vibration and noise control system must consider both the vehicle and the track as a working unit. Different geographic portions of the route may require different approaches so as to meet the goals identified in the project’s environmental clearance documentation. Chapter 9 provides guidelines with respect to trackwork design for low noise and vibration and introduces various concepts in noise and vibration control. 4.5.12 Signal/Train Control System Although the design of the signal control system does not greatly impact overall ballasted track design, it can affect specific parts of the design. The prime example of this interrelationship is the need for the insulated joints in the running rails, including associated impedance bonds to accommodate train control requirements. Such joints are normally required at the extremities of

Track Structure Design 4-69 interlockings, at each end of station platforms, at-grade crossings, within individual turnouts and crossovers, and at other locations to be determined by the train control requirements. For additional information on transit signal work, refer to Chapter 10. 4.5.13 Traction Power Refer to Chapter 11 for detailed discussion on the interaction between track alignment and trackwork design and the traction power system. 4.5.14 Grade Crossings Track designers must develop an acceptable interface wherever streets and roads cross the light rail tracks at grade. At-grade crossing panel systems for ballasted track are manufactured as prefabricated units and made of either rubber, reinforced concrete in a steel frame, or wood. The concrete panels and the rubber panels, when used over concrete cross ties, are designed to be easily installed and replaced during maintenance of the track. Timber panels are generally used only with timber ties and are generally not easily removed. Timber panels are also generally not compatible with track electrical isolation systems. While the prefabricated concrete and rubber crossing panels are designed to resist leakage of low-voltage signal current, they are generally less effective at controlling stray traction power currents, particularly when subjected to harsh conditions such as brine from salt and other roadway de-icing products. All grade crossings must create a flangeway between the street paving and the rail. ADAAG requires that crossings for rail transit systems have flangeways no wider than 2.5 inches [63 millimeters]. Crossings used by freight trains may have flangeways that are 3 inches [76 millimeters] wide. While products are available for filling in the open flangeway with a compressible material, to date, no such product is sufficiently durable for public highway crossings that might see hundreds of rail vehicle movements daily. Some grade crossings are created by using flangeway timbers along both sides of the rails to form the flangeways and paving the remainder of the area with asphalt. Although this style (or the rubber equivalent) is not as durable as the prefabricated crossing panels, it may be quite adequate in the maintenance facility and storage track areas, provided that electrical isolation is not an issue. Two critical design elements of all grade crossings are adequate drainage for the track and keeping the debris and dirt from accumulating within and adjacent to the crossing. Storm water runoff and debris from the street must be directed away from the track section, and the track must be designed with perforated pipe drains to keep the trackbed dry. Additional stabilization of the subgrade with geo-synthetic materials may be very cost-effective in reducing track surfacing costs. Failure to provide good drainage will result in pumping track and broken pavements. Road crossings tend to accumulate dirt and debris washed off the roadway or blown along the track. The debris accumulation results in fouled ballast and a path for stray current leakage. Accumulations of dirt and debris next to the rails and just beyond the ends of the crossing must

Track Design Handbook for Light Rail Transit, Second Edition 4-70 be cleaned out by a continuous maintenance program; otherwise, both stray current leakage and signal system malfunctions will develop. Due to the superior subgrade at most at-grade road crossings, the transition from ballasted track to the ballasted roadway track becomes a factor. There is a differential in track modulus or track stiffness that affects ride quality. Transition slab design at ballasted track roadways may be a requirement. Refer to Article 4.4.3.1 for transition information. The use of embedded track at grade crossings provides a very durable and reliable crossing. Embedded track provides a virtually maintenance-free and long-lived installation with excellent electrical isolation properties for the rail and a very smooth road crossing surface for automobile traffic. However, the higher track modulus of embedded track may dictate the need for a transition slab track segment. First cost will therefore likely be appreciably higher than an “off- the-shelf” modular crossing system designed without stray current control in mind. A life cycle cost analysis of crossing surface alternatives should consider the somewhat intangible impacts on both transit service and the community of frequent crossing repair and reconstruction. Coordination with the street design is also necessary to match the normally crowned street cross section with the profile of the tracks, which are not necessarily either level or even on a straight gradient. Particularly when the track grade is appreciable, it is extremely important to contour the approach pavement so as to channel storm water runoff and the associated debris it carries away from the track crossing structure and into appropriately designed street drainage systems such as curbside catch basins. To do so may require that the intersecting roadway be reconstructed/regraded for some appreciable distance from the track. 4.6 DIRECT FIXATION TRACK (BALLASTLESS OPEN TRACK) Direct fixation (DF) track is the most common LRT trackform for use on aerial structures and in tunnels. It is also often used in areas where it would be difficult to maintain ballasted track in proper alignment and surface. 4.6.1 Direct Fixation Track Defined Direct fixation track is a “ballastless” track structure in which the rail is mounted on direct fixation fasteners that in turn are anchored to an underlying concrete slab. The slab could be a slab on grade, an aerial structure deck surface, or a concrete tunnel invert. Direct fixation track is also used for construction of at-grade track under unusual circumstances, such as when there is a relatively short segment of at-grade track between two direct fixation track structure decks. Direct fixation track can require only minimal maintenance if it is installed according to design and with a high standard of construction quality and precision. Just as with any other trackform, several vehicle/track-related issues must be resolved prior to developing a direct fixation track design. These issues relate to the vehicle’s wheel gauge, wheel profile, and truck axle spacing design; the track gauge and rail section; and satisfactory operational compatibility of the vehicle with the guideway geometry. Acoustic concerns are also very important to consider with noise and vibration mitigation measures, as discussed in Article 4.6.8

Track Structure Design 4-71 4.6.2 Direct Fixation Track Criteria To develop direct fixation track design, the following track components and standards must be specified: • Rail section. • Track gauge. • Guarding of curved track and restraining rail features. • The type of direct fixation track structure to be used: − Direct fixation rail fastener installation on raised reinforced concrete plinths. − Direct fixation rail fastener installation on thin cementitious or epoxy grout pads. − Booted tie installation. − Plinthless direct fixation track installation. If direct fixation rail fastener construction is selected, the type of rail fastener and supporting structure to be employed beneath that fastener must be determined. The principal such details are reinforced concrete plinth, cementitious grout pads, booted tie blocks in a structural slab and a “plinthless” design that connects the fasteners directly to a structural substrate. Each of these will be discussed in Article 4.6.3. 4.6.2.1 Direct Fixation Track Rail Section and Track Gauge Refer to Article 4.2 and Chapter 5 of this Handbook for determination of rail section, track gauge, and flangeway requirements. 4.6.2.2 Direct Fixation Track with Restraining Rail Refer to Article 4.3 to determine the requirements, locations, and limits for guarding track with restraining rail. 4.6.2.3 Direct Fixation Track Rail Fasteners Refer to Chapter 5, Article 5.4, to determine the requirements for specifying direct fixation fasteners and shims. 4.6.2.4 Track Modulus Direct fixation track is typically much stiffer vertically than ballasted track. This rigidity must be attenuated if transmission of noise and vibration is to be avoided. Careful design selection of an appropriate track modulus and specification of an appropriate spring rate for the direct fixation rail fastener must be made in accordance with both Article 4.3 of this chapter and Chapter 9 of this Handbook. The expected vehicle dynamics/harmonics must be considered. 4.6.3 Direct Fixation Track Structure Types The very earliest form of direct fixation track was effectively timber tie track with the ballast replaced by plain, non-reinforced concrete. Such designs were very common not only on subway and elevated rail transit lines, but also in railroad terminal stations and were the standard detail for such areas from the late 19th century up through about 1960. Many of these early installations

Track Design Handbook for Light Rail Transit, Second Edition 4-72 are still in service. The design was simple: timber cross tie track was constructed in skeleton form, blocked up to grade and alignment, and then the lower portions of the cross ties were encased in concrete, locking the track structure in place. Often, only every fourth or fifth tie would be a full-length cross tie for holding gauge, and the intermediate ties would be short timber blocks supporting only a single rail plate. Such designs incorporated no specific measures to control stray traction power currents or ground-borne vibrations. Encased timber tie track is no longer constructed except in very limited circumstances for maintenance of existing systems. Modern designs of direct fixation track construction include the following designs: • Concrete Reinforced Plinths: This form of direct fixation track constructs rectilinear reinforced concrete blocks or plinths that support several direct fixation fasteners under a single rail. The plinths can vary in length and typically support between three and six fasteners, although longer plinths supporting up to 12 or more fasteners have been used. Such long plinths used to be the norm, since they minimized formwork, but long plinths are now discouraged because of problems with transverse shrinkage cracks. The periodic chases between plinths allow for cross track drainage into a trough that is typically located either on the centerline of track or, in the case of an aerial structure, along the structure’s centerline between the two tracks. The chases also accommodate transverse conduit and cabling requirements of the traction power and train control systems. • Cementitious Grout Pads: This form of direct fixation track mounts each individual rail fastener on an individual cementitious concrete grout pad. The fasteners are held in place by anchor bolt assemblies (preferably a female insert type) that are grouted into holes cored through the grout pad into the supporting concrete base. Thin layers of cementitious grouts can be problematic. Alternate grout pad material such as polyurethane can be considered in the design to suit specific site conditions. Particularly for very thin pads, the polyurethane product may provide much better surface adherence and a more durable pad. However, polyurethane grouts are subject to very strict mixing and handling procedures; not following the procedures could result in an inferior product. See Chapter 13 for additional discussion of construction issues related to grout pads. • Ballastless Booted Tie Blocks: This form of direct fixation track is an updated version of the original encased timber tie design. It typically incorporates two block concrete cross ties that have an elastomeric “boot” encasing the lower portion of each tie that provides electrical and acoustic isolation between the concrete tie blocks and the encasing concrete. As with the original encased timber tie design, most ties would be single blocks with no cross tie member between the rails. • “Plinthless” direct fixation track has no secondary reinforced concrete plinth or grout pad beneath the rail fastener. In this form of direct fixation track, the rail fastener is mounted directly to an underlying slab, typically the deck of an aerial structure. The fastener anchor inserts are cast directly into the deck slab at the time of structure deck fabrication. Variations of the above designs can be found, such as direct fixation rail fasteners bolted directly to structural steel bridge members. Such arrangements are generally in response to a site- specific design issue and will not be addressed in this Handbook.

Track Structure Design 4-73 4.6.3.1 Reinforced Concrete Plinths The most common direct fixation track design is the raised reinforced concrete plinth system. The authors of this Handbook strongly recommend the use of plinths for most direct fixation track installations. Reinforced concrete direct fixation plinths serve many purposes: • The variable height of the plinth is used to compensate or eliminate discrepancies in the elevation and cross slope of the underlying structure deck, at-grade track slab, or tunnel invert—a benefit that is required in most constructions. This benefit is particularly valuable on aerial structures, where the as-built tolerances in the deck elevation, due to girder camber, might require abnormally thin or thick grout pads if that technique were proposed. • The method of embedded dowels/stirrups in the parent deck, slab, or invert followed by the addition of a reinforcing bar plinth cage provides a permanent mechanical connection, ensuring the longevity of the plinth installation. • Plinth construction is particularly conducive to the “top-down” construction method, which very nearly eliminates the need for secondary corrective height shimming. • The second-pour concrete plinth is an ideal method of providing the amount of superelevation required at each curve. Plinth height can be continuously and accurately graduated, providing a smooth transition to superelevation. The reinforcing bar cages can be of graduated design to compensate for plinth growth. • The bottom surface of the concrete plinth can be an easily defined dividing point between two different construction contracts—the first building the initial underlying structure and the second building the trackwork installation—providing a clean division of responsibility. • The height of the concrete plinths permits generous openings beneath the rail for random signal and traction power cable installations with minimum impact to both disciplines. • The height of the concrete plinth provides ample clearance beneath the rail for storm water runoff and minimizes problems of standing water on flat trackways. The raised plinths minimize the possibility of water pooling against the rail fasteners and possibly compromising their electrical isolation. Some designers object to the plinth design because it places the top-of-rail elevation about 14 inches [360 millimeters] above the invert. They argue that in the event of a derailment where the wheels do not end up on top of the plinths, substantial damage to the underside of the rail vehicle could result. This is virtually a moot point since, unlike railroad freight cars, LRV truck designs rarely provide enough underclearance to allow the wheels to drop to even the base of rail elevation, much less the top of the plinths. Hence, the potential damage would likely be the same regardless of which direct fixation (or ballasted) trackform were used. Only embedded track would limit damage to the underside of the vehicle in the event of a derailment. The reinforced concrete plinths used for direct fixation track include various designs to suit tangent track, curved track, superelevated track, and guarded track with restraining rail. The

Track Design Handbook for Light Rail Transit, Second Edition 4-74 direct fixation track designs affect the lengths and shapes of the plinths and the reinforcing bar configurations as follows. 4.6.3.1.1 Concrete Plinth in Tangent Track Concrete plinths in tangent track generally follow one of two designs, both shown in Figure 4.6.1: • Concrete plinths of sufficient width and height mounted directly on the top of the concrete deck, slab, or invert. • Concrete plinths of sufficient width and height installed within a recessed opening in the concrete deck, slab, or invert. 4.6.3.1.1.1 Concrete Plinth on Concrete Surface. The concrete plinth width and height must be sufficient to accept the full length of the fastener and anchor bolt insert height. It must also accommodate the reinforcing steel that is required to hold the plinth concrete to the concrete surface and confine the concrete mass that supports the direct fixation rail fastener and anchor bolt insert. The concrete plinth is rigidly connected to the deck, slab or invert concrete surface with a series of stirrups or dowels protruding from the deck or invert. The connection is made through a plinth reinforcing steel bar cage that passes under the stirrups to lock down the plinth. The dowels must extend a substantial depth into the underlying concrete to obtain holding force. Some designs, instead of depending solely on the pullout strength of the embedded dowel, include a horizontal bent leg so the dowel can be welded to the reinforcing bar system in the underlying slab. However, this design complicates the finishing of the concrete slab surface. Figure 4.6.1 Concrete plinth design—tangent direct fixation track The fastener anchor bolt inserts may be installed by the cast-in-place method or drilled and epoxy grouted in place. Cast-in-place installation (top-down construction) is recommended as it results in less disturbance to the concrete plinth and eliminates any possible problems with drilling through reinforcing steel. Top-down construction also eliminates the extra work and potential problems of dealing with the epoxy grout materials used in the core drilling placement method.

Track Structure Design 4-75 4.6.3.1.1.2 Concrete Plinth in Concrete Recess. The concrete plinth design has a variant wherein the second-pour concrete can be recessed into a shallow trough in the base concrete slab. The recessed design allows a reduced plinth height above the deck, slab, or tunnel inverts and provides additional side bonding by keying in the four sides of the plinth. The recessed design obviously requires that a trough be formed in the trackway deck, slab, or invert, an additional work activity and hence expense to the contractor building the trackway (especially in curved track areas). The extra cost associated with forming the trough is not insignificant, and designers should carefully weigh the costs and benefits of the recessed design before deciding on a preferred method. The trough may affect the structural integrity of the deck or slab, particularly on aerial structures, so the recessed design must be coordinated with the structural design team. 4.6.3.1.2 Concrete Plinth in Superelevated Curved Track Concrete plinth design for curved track must consider track superelevation. The track designer must provide guidance to the construction contractor for setting the height of the plinth formwork so that the required gradual introduction/elimination of superelevation in the spiral areas and the constant superelevation in the central curve area are achieved. In addition, care must be taken to ensure that the top-of-rail plane rotation and the parallel top of concrete plinth rotation at the low rail allow sufficient vertical height for the inside, or closest, anchor insert to the curve center radius point. The plinth height is established at the elevation of the low inside rail of the curved track, as shown in Figure 4.6.2. Applying the profile grade line elevation at the low rail of the curve, the superelevation is established by rotating the top-of-rail plane around the centerline of the low rail head. The addition of superelevation alters the track cross slope and the thickness of the concrete plinths so that the typical track section is no longer symmetrical. The embedment of the field side anchor bolt insert of the low rail fastener establishes the height of the plinths. In areas of high superelevation, the plinth height should be closely coordinated with the structural designers so as to be certain that the structural deck, slab, or invert is low enough to accommodate the anchor assemblies without requiring chipping of the invert to fit the insert height. The reinforcing bar requirements and configurations should be treated as a special series of graduated bar shapes that suit the variations in the plinth heights, as shown in Figure 4.6.2. Recessing the plinths into the deck surface can be particularly advantageous in superelevated curved track since it can substantially reduce the plinth height at the high rail when entire superelevation rise is developed by the height of the plinth. Refer to Article 4.6.3.1.5 for additional discussion of plinth heights. 4.6.3.1.3 Concrete Plinths with Restraining or Emergency Guard Rail The use of either a restraining rail or an emergency guard rail in direct fixation track will require that the concrete plinths be wider than normal. Figure 4.6.3 illustrates a typical wider plinth for installation of restraining rail. A similar wide-plinth arrangement is required for an emergency guard rail. Again, this concrete plinth arrangement can be either mounted directly to the surface or the recessed opening in the concrete deck, slab, or invert.

Track Design Handbook for Light Rail Transit, Second Edition 4-76 Figure 4.6.2 Concrete plinth design—graduated J-bars to match superelevated plinth heights

Track Structure Design 4-77 Figure 4.6.3 Concrete plinths—superelevated track with restraining rail 4.6.3.1.4 Concrete Plinth Lengths Concrete plinths can be formed in various lengths. Typical plinths of intermediate lengths will accommodate three to six direct fixation fasteners between drainage chases, as shown in Figure 4.6.4. Figure 4.6.4 Concrete plinth lengths

Track Design Handbook for Light Rail Transit, Second Edition 4-78 Concrete plinth lengths are dependent on several track design factors: whether the track is tangent or curved, whether formwork in curved track is curved or chorded, and the locations of construction joints and expansion joints in the deck, slab, or invert. Concrete plinths in curved track are generally constructed in short tangent segments for ease of formwork. The mid- ordinate offset of the alignment must be considered when determining tangent plinths to provide ample anchor insert clearance and fastener bearing to the top of the plinth. Concrete plinth lengths are affected by differential shrinkage of structure and plinth, local climate conditions, and temperature ranges. Longer plinth sections, accommodating 7 to 15 fasteners, have been installed. However, these designs have had significant problems with concrete shrinkage and cracking and are therefore no longer recommended for new design. To reduce or stop the potential of hairline cracks generating from the anchor insert positions during top-down construction due to thermal movement of the attached rail, the rail should be unclipped from the rail fasteners as soon as the plinth concrete has taken an initial set. This will typically require the use of special temporary rail clips as the elastic rail clips provided with most direct fixation rail fasteners require application of force for clip removal, which could also crack the green concrete. For more information on direct fixation construction, refer to Chapter 13. 4.6.3.1.5 Concrete Plinth Height The heights of the rail section, the direct fixation fastener with insulating shim, the length of the anchor bolt insert, and the minimum overall height of the plinth (generally 6 inches [150 millimeters] under the centerline of the rail) must be determined to establish the overall height of the direct fixation track structure. The plinth height/thickness should generally be no less than 1 inch [25 mm] greater than the length of the anchor inserts, but other factors will sometimes require a larger dimension. To facilitate drainage, the surface of the bridge deck, at-grade slab, or tunnel invert should generally be sloped at 1:40 to 1:80 toward the centerline of the track or structure. On single-track installations of a slope of 1:40 toward the centerline of track and on double-track installations a slope of 1:80 toward the centerline between both tracks on double-track installations would provide the necessary runoff. These slopes will affect the height of the direct fixation plinths. In addition to lateral drainage slope, the longitudinal surface drainage gradient is critical to provide adequate drainage of the trackbed. A track gradient of zero percent is not desirable due to the possibility of surface standing water. If the structure gradient is less than about 0.5%, the structure cross slope becomes critical to keeping the track area dry. The key dimension to establishing the plinth height is the dimension from the top-of-rail plane to the intersection of the deck or invert slopes at the track centerline. The plinth heights should be kept to a minimum to enhance structural stability, especially if the deck or invert is relatively level and the track alignment requires a high amount of superelevation at the outside rail. See Article 4.6.3.1.1.2 for discussion of the advantages of recessing the plinth into a key in the underlying slab.

Track Structure Design 4-79 4.6.3.1.6 Plinths on Decks Twisted for Superelevation Several rail transit projects since 2000 have used segmental concrete girders where track superelevation has been achieved by twisting the deck. This allows all plinths to be the same height, as they are in tangent track. If this method is used, it is recommended that the pivot point be placed at the centerline of the structure between both tracks and that the profile grade line of the structure be in the plane of the tops of the rails. This arrangement depresses the profile of the inside track of the curve below the profile grade line of the structure. Similarly, the outer track’s profile will be above the structure’s grade line. Hence, there will be three distinct profile grade lines—one for the structure and one for each track. This method results in induced superelevation as the tracks rise or fall relative to the profile grade line of the structure. This extra vertical translation must be accounted for in the determination of the appropriate spiral lengths. 4.6.3.1.7 Direct Fixation Vertical Tolerances It is possible to construct plinths to very tight vertical tolerances. Nevertheless, the finish elevations of the seats for the individual direct fixation rail fasteners (and hence the top of rail) is critical to vehicle ride quality and interaction between rail and track structure. To achieve a near- perfect track surface longitudinally and to avoid situations where some fasteners are stretched vertically to match the position of the rail, the use of shims between the top of the plinth and the base of the direct fixation fastener is customary. The maximum difference in elevation between adjacent fasteners should be less than 1/16 inch [1.5 millimeters], probably the thinnest practical shim thickness. Shims can be as thick as ½ inch [13 millimeters]. Thicker shims are occasionally used, but should be entirely unnecessary for new construction if the plinths are constructed correctly. Well-constructed track using the top-down method often requires no shims for vertical adjustment. Some designs use a minimum of one high-density polyethylene (HDPE) shim (typically ⅛ inch [3 mm]) so as to provide an additional layer of electrical isolation. See Chapter 13 for detailed discussion of the top-down construction method. Occasionally, thicker shims might be required to restore the track profile if the structure itself settles. In such cases, extra-length anchor bolts may be required so as to maintain the proper amount of thread engagement. 4.6.3.1.8 Concrete Plinth Reinforcing Bar Design Plinth reinforcement begins with the construction of the trackway structure deck, slab, or tunnel invert. A series of stirrups or dowels is embedded in the underlying slab, placed longitudinally within the footprint of the concrete plinth, positioned to clear the embedded anchor bolt inserts and the ends of plinth openings or gaps. The stirrups should protrude a minimum distance of 2 inches [75 millimeters] from the deck, slab, or invert surface to allow both the formed transverse reinforcing steel and the plinth concrete to lock under the stirrups. The stirrup height must be designed to suit the eventual concrete plinth height and reinforcement design. Very often, an entirely different contractor will construct the slab or bridge deck upon which a subsequent track contractor will build the direct fixation track. The structure or invert contractor is normally responsible for the proper placement of the initial stirrup/dowel reinforcing steel that projects from the deck or base concrete. This projecting reinforcing steel must be properly installed and protected from construction traffic damage after installation. The wheels of construction equipment often damage stirrups. The use of the recessed plinths may help mitigate this problem. Refer to Chapter 13 for additional information on construction of direct fixation track.

Track Design Handbook for Light Rail Transit, Second Edition 4-80 The plinth reinforcement that is installed by the trackwork constructor consists of a series of “J- hook” bars and longitudinal bars. A transverse collector bar is sometimes placed at the ends of each concrete plinth for stray current control as shown in Figures 4.6.5A and 4.6.5B. Figure 4.6.5A Concrete plinth reinforcing bar details

Track Structure Design 4-81 Figure 4.6.5B Concrete plinth reinforcing bar details (continued) The design size of the concrete plinth will determine the lengths and bend radii of the “J” hooks and the length of the longitudinal bars. Tangent track will require J-bars of a uniform height to conform to the general height of the concrete plinth. Curved track alignments with superelevation will require various sizes and shapes of reinforcing bar “J” hooks as shown in Figures 4.6.2 and 4.6.3. Design size of reinforcing bars and stirrup locations must allow for providing 1 ½ inches [38 millimeters] minimum of concrete cover from the edge of the bar to the face of the concrete and a ¾-inch [20-millimeter] clearance to the face of the fastener anchor bolt inserts. To combat potential stray current leakage or flow within the concrete plinth, two distinct design concepts exist. These concepts are the following: • All of the reinforcing steel is made 100% electrically continuous through a welding process at every location where bars touch or overlap. • The reinforcing steel is made 100% electrically discontinuous by using epoxy-coated bars and diligently patching the coating at all cut bar ends and at any chips or other damage that occurs during construction.

Track Design Handbook for Light Rail Transit, Second Edition 4-82 The concrete plinth reinforcing bar system can be made electrically continuous by following these steps: • The deck or invert stirrups installed during the initial construction must be connected (welded) to the deck or invert reinforcing bar network. • The concrete plinth reinforcing bar system must be completely connected (welded) to the protruding deck or invert stirrups. • When the stirrups or dowels are not connected (welded) to the deck or invert reinforcing bar system, then the individual concrete plinth reinforcing bar networks must be completely connected (welded) and connected to a negative ground system. This requires connections between each plinth at the concrete plinth openings or gaps. The concrete plinth reinforcing bar system can be isolated by the following method: • The use of epoxy-coated reinforcing bars in the stirrups and the concrete plinth reinforcing bar network provides the required stray current corrosion protection. • Care must be exercised during construction to retain complete protective epoxy-coating coverage on the stirrups and concrete plinth reinforcing bar network. Chipped or damaged epoxy coating must be covered by an acceptable protective paint that is recommended by the epoxy-coating manufacturer and compatible with the initial epoxy- coating material. Refer to Chapter 8 for additional information on direct fixation track stray current mitigation methods. Refer to Chapter 13 for additional discussion on direct fixation plinth construction methods. In some cases, surface water can penetrate the joint between the plinth concrete and the base concrete, causing corrosion of the stirrups. In tunnels that do not have adequate means of leak control, the potential for surface water to penetrate the separation point may be unavoidable, leading to reinforcing bar rusting and corrosion. Various sealants, such as epoxies, have been used in the attempt to seal this joint, but virtually every product available will eventually dry out, harden, and peel away. The use of a sealant can actually exacerbate a seepage condition by trapping water beneath the plinth concrete. As a guideline, sealants are discouraged and the use of epoxy-coated reinforcing steel for stirrups is recommended. 4.6.3.2 Cementitious Grout Pads Cementitious grout pad track designs include • Short cementitious grout pads of sufficient width to allow for installation of the direct fixation fastener that is formed and poured directly to the concrete deck or invert. A typical configuration is as shown at the left rail in Figure 4.6.6. • Short cementitious grout pads mounted within a recessed opening in the concrete deck or invert, as shown at the right rail in Figure 4.6.6.

Track Structure Design 4-83 Figure 4.6.6 Cementitious grout pad design—direct fixation track Grout pads typically support only a single fastener, although current practice is to build longer pads to support at least four fasteners. The longer design provides improved integrity of the pads and ease of maintenance if a fastener is replaced or needs to be repositioned. Grout pads can also be formed out of a selective polyurethane product that is proven to be successful if administered properly. 4.6.3.2.1 Cementitious Grout Pad on Concrete Surface The relatively thin/short cementitious grout pad design acts as a leveling course between the underside of the direct fixation fastener and the concrete deck or invert surface. This design requires core drilling of the concrete structure deck or concrete tunnel invert to grout the anchor bolt or female anchor insert in place. The drilling can be undertaken either prior to or after grout pad installation. The bolt assemblies are permanently anchored with an epoxy grout material. The grout pad itself may not provide any lateral stability to the rail fastener anchorage system. The actual anchorage of the direct fixation rail fastener is achieved by penetration of the anchor bolts or inserts into the underlying structural deck slab, tunnel invert, or at-grade track slab. Depending on the thickness of the grout pad, it may be necessary to use either taller anchor inserts or longer threaded anchor rods than would be used with concrete plinths. Coordination is required with the structural designers concerning the locations of the reinforcing steel in the underlying slab so that the bars are located clear of, but not too distant from, the lines of core- drilled holes. The cementitious grout pad can be formed and poured before the rail fastener is placed; however, it may be difficult to achieve an absolutely level and true top surface for the rail fastener that sits uniformly in a longitudinal plane surface (parallel to the profile grade line) with the adjacent fasteners. If the grout pad is slightly too high, corrective grinding may be required. If it is too low, it may be necessary to place metallic or polyethylene shim(s) beneath the rail fasteners.

Track Design Handbook for Light Rail Transit, Second Edition 4-84 Alternatively, the assembled rail and rail fasteners can be suspended at proper grade and alignment above the concrete invert and the grout either pumped, injected, or “dry packed” under the rail fastener. If this approach, known as top-down installation, is taken, it is essential to ensure that the grout does not enter the recesses on the bottom surface of the direct fixation rail fastener because this could compromise the rail fastener spring rate. This can be avoided by placing a temporary shim beneath the direct fixation rail fastener before grout placement. It will also be necessary to lift the rail and fasteners after the grout has cured to remove the temporary shim and locate and fill in any voids or “honeycomb” in the top surface of the grout pad that are caused by trapped air or improper grout placement. The temporary shim is then often replaced with a high-density polyethylene (HDPE) shim for additional electrical insulation. Grout pads typically depend on the strength of the bond between the concrete deck, slab, or invert and the grout for their stability. Reinforcing steel typically cannot be used because the pad is so thin. The use of fiber reinforcing might be possible with some grout mix designs. The concrete invert is typically roughened before placement of grout. Epoxy bonding agents used to be specified between the grout pad and the concrete surface to enhance bonding. However, due to the different thermal expansion characteristics of epoxy and concrete, this practice is no longer recommended. 4.6.3.2.2 Cementitious Grout Pad in Concrete Recess Some transit systems have experienced grout pad delamination, because cementitious grout pads have a tendency to curl or pull away from the parent concrete deck or invert during curing and especially aging. It is possible to achieve better bonding with less likelihood of such failures by forming the grout pad within recesses in the concrete deck, slab, or invert. The recessed design provides additional deck, slab, or invert bonding by locking in the four sides of the grout pad. The anchor bolt assembly drilling can be undertaken either prior to or after grout pad installation. Prior drilling in the parent concrete and epoxy grouting the anchor insert base in place is recommended as it results in less disturbance to the bond of the cast-in-place grout pad. The grout pads would be placed after the anchor inserts have been permanently secured in place. 4.6.3.2.3 Cementitious Grout Material The selection of a cementitious grout material must be undertaken carefully. The use of incompatible special epoxy grouts and additives to the epoxy grout material can result in eventual pad delamination and cracking. The material should be compatible with the structure deck or tunnel invert concrete and have similar thermal expansion characteristics. The cementitious grout material must also be compatible with the service environment of the trackway. Large deviations in the as-built elevations of the concrete invert can result in very thin grout pads. Track superelevation can result in very thick grout pads. Both can be troublesome, but thin pads are particularly prone to early failure. Cementitious grout pads that are less than 1 ½ inches [38 millimeters] thick are generally more susceptible to fracture. As a guideline, although the cementitious grout pad design has been and is currently used on some transit systems, it is not recommended due to the design’s history of pad failure. Cementitious grout pads tend to delaminate and break down, requiring high maintenance, particularly in colder

Track Structure Design 4-85 climates that are subject to freeze-thaw cycles. Abbreviated height grout pads make installation of signal cables, conduits, and traction power bonding cables more difficult. In wet and dirty tunnel environments, the reduced distance between the tunnel invert and the base of rail can lead to significant stray current leakage. Locations with minimal overhead clearance, which therefore require a low-profile direct fixation track structure, may be the best application of the cementitious grout pad system. 4.6.3.3 Direct Fixation “Ballastless” Concrete Tie Block Track One alternative to the fastener-on-plinth direct fixation track system is the use of independent, embedded, dual-block concrete ties in rubber boots as shown on Figure 4.6.7. Versions of this type of installation and its predecessors date back to the mid-1960s, and it is essentially a descendant of the embedded tie trackform discussed in Article 4.6.3. This system can provide a track that is appreciably softer than the previously discussed direct fixation trackforms, and one vendor markets its version under the trade name of “Low Vibration Track” or “LVT.” LVT and similar trackforms are marketed as a direct equivalent to direct fixation track that uses resilient rail fastener plates. [3] Figure 4.6.7 Independent dual-block concrete tie track system Earlier versions of this type of dual-block concrete tie track incorporated a steel angle between the concrete blocks to hold gauge. Current designs do not include these gauge bars since the concrete encasement holds track to gauge. The individual concrete tie blocks support and anchor the rail. Microcellular elastomeric pads support the blocks. These base pads and the tie blocks are enclosed in a tub-shaped rubber boot before installation. The microcellular pad provides most of the track’s elasticity. A rail seat pad also provides some cushioning of impact loads, although in one case it was found that the rail pad design acted in resonance with the underlying microcellular pad, resulting in excessive rail corrugation. Embedded dual-block tie track can be engineered to provide whatever track modulus or spring rate is required by changing the composition or thickness of the microcellular pad. The most common application has a spring rate in the range of 90,000 to 140,000 lb/inch [15,760 to 24,500 N/mm] to provide maximum environmental benefits.

Track Design Handbook for Light Rail Transit, Second Edition 4-86 The electrical barrier for encased tie, direct fixation track systems is provided at the rail base. Similar to concrete tie fastenings, the electrical barrier is established by an insulated resilient rail seat pad and spring clip insulators. The tie boot provides a secondary barrier for electrical isolation. Most encased tie systems reduce the need for reinforcing steel. LVT does not require a reinforced invert, which often makes this system competitive with a plinth type of installation. The installation of LVT—and almost all encased tie systems—requires top-down construction, where the rail is suspended from temporary supports, with tie blocks and rail fastenings attached, at the final track horizontal and vertical alignments. The encasement concrete is then poured into the tunnel invert around the tie blocks, locking the track in place. When the concrete is cured, the supports are removed. Special details can be incorporated in the tie block rail fastening system so that lateral and vertical adjustments can be made after the encasement concrete has been poured. Encased tie systems vary widely in cost, but can usually be installed quite rapidly compared to plinth or grout pad type systems. Tie block replacements are feasible on a small scale, consisting of a slightly smaller concrete tie block grouted in the open cavity of a removed tie block. 4.6.3.4 Plinthless Direct Fixation Track One of the key design factors for direct fixation track, particularly on aerial structures, is compensating for the as-built tolerances for the underlying concrete base, be that a bridge deck, a slab at-grade, or a tunnel invert. Typically, such large concrete pours will have finish construction tolerances that are much coarser than can be tolerated if a smooth top-of-rail elevation is to result. Aerial structures require additional consideration since the as-built camber of the underlying superstructure cannot always be predicted with certainty, particularly in the case of bridges built with concrete beams, such as the common AASHTO girders. The decks of such bridges could easily vary by 1 to 3 inches [2 to 7 cm] from the design elevations. While such coarseness can often be tolerated in a highway bridge, it’s not satisfactory as a surface for mounting of direct fixation rail fasteners. The direct fixation track detail therefore needs to be able to compensate for these structural tolerances. Plinths and grout pads, as previously discussed, provide the track constructor with practical means to make such adjustments. Advances in the design and construction methods for segmental concrete bridges has led to an ability to very tightly control the installed finish elevation of such superstructures. Tolerances appreciably less than 1 inch [25 mm] have reportedly been achieved. This raises the possibility of directly mounting the direct fixation fasteners to the superstructure deck, making final elevation adjustments by the use of shims between the bridge deck and the rail fastener. This method could result in construction cost savings, due to the elimination of the need to form plinths or grout pads and a slight reduction in the dead load of the overall structure. Nevertheless, “plinthless” direct fixation track should not be proposed lightly. This design requires very serious consideration of issues such as the following: • Track geometrics, particularly in curved track zones, where, in addition to the profile issues discussed in Article 4.6.3.1.6, it will be necessary for the spirals in parallel tracks

Track Structure Design 4-87 to begin and end at virtually the same point along the structure so as to match the superelevated bridge deck. • Extremely precise placement of the anchor inserts in the bridge segments at the casting yard, keeping in mind that each segment could be up to 30 feet [9.1 meters] wide. • Extreme accuracy in the structure deck finish to allow for installation of the rail fastener plate and obtaining the proper rail cant with minimal shimming. • Precise inclination of the erected superstructure to achieve zero cross level in tangent track and proper superelevation in curved track, including spirals. • Possible extensive fastener shimming to correct profile grade line, particularly if the as- built tolerances for the bridge deck elevation are something less than hoped for. This could require longer bolts to accommodate large shim thicknesses and raises questions concerning bolt thread engagement and higher bolt thread root stresses in extreme cases. • Possible non-standard anchor insert design to accommodate a longer threaded section of longer anchor bolts. • Little or no space beneath the rails for electrical system conduits and cables, with the possibility of impaired storm water drainage of the bridge deck. Impaired storm water drainage could result in ponding of storm water and accumulation of sediments around the direct fixation rail fasteners, which could result in degraded electrical isolation. In addition, plinthless track is impractical in areas of special trackwork. As of this writing, in 2011, at least one rail transit project has been constructed with a significant length of plinthless aerial structure track (Vancouver), and another large project (Honolulu) is currently under construction. The long-term success of these installations of these projects will be a matter of great interest. 4.6.4 Direct Fixation Fastener Details at the Rail Typically, the track system will have the rail positioned with a cant of 1:40 toward the track centerline. Rail cant in direct fixation track may be achieved by two methods: • The top surface of the concrete plinth or grout pad can be sloped to match the required cant. In such cases, the direct fixation rail fastener itself would be flat, with no built-in rail seat cant. • The plinth concrete or grout pad can be poured level (or parallel with the plane of the top of rails in superelevated track) and the rail fasteners can be manufactured with the desired cant built into the rail seat of the fastener. Both methods can produce acceptable results. Placing the cant in the rail seat of the fastener simplifies the construction of plinth formwork in tangent track, as no slant is required in the formwork, and better ensures that the desired cant will actually be achieved, particularly when bottom-up construction is anticipated. If top-down construction is used, rail cant can be reliably achieved if the jigs used to support the assembled rails and rail fasteners incorporate cant adjustment capability regardless of whether canted or non-canted fasteners are used. Note that if canted fasteners are used in the main track, it may still be necessary to procure non-canted fasteners for use in special trackwork areas. Also, placing the cant in the plinth better ensures

Track Design Handbook for Light Rail Transit, Second Edition 4-88 that storm water will drain off the surface of the plinth, particularly in track that is longitudinally level or only on a slight gradient. Lateral adjustment capability and fastener anchor bolt locations are important elements in the design and configuration of direct fixation rail fasteners. The rail cant location must be considered when positioning embedded anchors. Rail cant at the base of rail or at the top of the concrete alters the anchor positions (refer to Figure 4.6.8), and this factor must be considered when using any sort of bottom-up construction method. Excessive shimming on a canted concrete surface may shift the rail head closer to the centerline of track, which narrows the track gauge. For additional information on direct fixation fasteners, see Chapter 5. Figure 4.6.8 Rail cant and base of rail positioning 4.6.5 Direct Fixation Track Drainage Drainage is as important to the success of a direct fixation track installation as it is to any other type of track structure. This includes drainage of runoff water from the top surface of the track and the subsurface support system. When designing the drainage system, the trackwork designer must also consider other system installations, such as signal and traction power conduits, that may also need to occupy parts of

Track Structure Design 4-89 the invert. Close coordination as to interfacing design between track and system designers of disciplines is required to be certain that conduits, cabling, and the racks that support them do not block surface deck runoff and are not positioned so close to the deck that they will trap wind- blown debris, thereby creating dams and standing water. Conveying this information to the trades that actually install these electrical systems can be a challenge since the system designers typically do not provide that level of detail in their drawings. Refer to Chapters 10, 11, and 13 for additional information on design and construction of direct fixation track with other disciplines to be certain drainage is not compromised. Direct fixation track built on a bridge structure will obviously not have to directly contend with any subsurface drainage issues. Direct fixation track constructed at-grade or in a tunnel, on the other hand, must be properly drained beneath the track slab. Standard underdrain details, similar to those used in highway design, must be provided to keep groundwater out of the undertrack area. The successful direct fixation track will include an efficient surface runoff drainage system. Experience has shown that foresight in the design of surface drainage for the direct fixation track structure is required to avoid accumulation of standing water or trapped water pockets. Surface runoff from a direct fixation track area will carry with it debris and dirt that accumulate on any exterior concrete surface. This material, if allowed to flow into an adjoining ballasted track area, will foul the ballast and degrade the performance of the ballasted track. To prevent this, the interface of ballasted track to direct fixation track should include the following: • A transverse drainage chase or a diverting end wall designed to direct surface runoff to a deck drainage system or a sloped catch basin area. • An end wall extending from the deck surface up to the top of cross tie level so as to fully retain the ballast section. • Concrete plinths that do not butt up to the ballast end wall retainer. Lateral drainage chases between the last plinth face and the ballast retaining wall are essential. The design positioning of deck surface drainage scuppers must consider the rotation of the deck or invert due to superelevation. 4.6.6 Direct Fixation Stray Current Protection Requirements The track structure design requires an electrical barrier at the rail. Direct fixation track generally provides this electrical barrier within the direct fixation fastener body and on the surface. An insulating resilient material with a specified bulk resistivity forms the elastomeric and insulating portion within the fastener body. A minimum surface leakage distance of ¾ inch [19 mm] is recommended between ground and any part of the track structure that carries traction power current. A projecting insulating shim below the fastener increases surface leakage distance, providing additional isolation. Even if the above design features are implemented, a wet and dirty condition due to accumulated debris can quickly create alternate paths for stray current leakage. This condition is common in tunnels and can be devastating to the track in a damp tunnel. Transit agencies that have let this

Track Design Handbook for Light Rail Transit, Second Edition 4-90 situation get out of control have found it necessary to completely replace both direct fixation rail fasteners and rails that were damaged beyond salvage by corrosion. The most effective corrosion protection measure in such cases is likely a concerted housekeeping program that includes periodic power washing of the entire track structure to remove the contaminants that act as catalysts of corrosion in damp environments. Experience has shown that a thorough wash down has reduced stray currents to acceptable levels. It is important that this be done before corrosion has damaged the track structure. For more information on electrical barriers on direct fixation fasteners, see Chapter 5. 4.6.7 Direct Fixation Special Trackwork The direct fixation special trackwork portion of any transit system will require special treatment and a different concrete plinth design than main line direct fixation track. The supporting plinths or track slabs require detailed layout of the fasteners, switch rods, and gauge plates, careful consideration of paths for storm water drainage, plus close coordination with the signal and electric traction designers. Direct fixation special trackwork in contemporary light rail transit systems generally consists of junctions with turnouts and crossing diamonds and pairs of turnouts grouped to act as single crossovers for alternate track operations. Either operating requirements or space limitations may dictate the installation of a double or “scissors” crossover with four turnouts and a crossing diamond. Using double crossovers in tunnels and on bridges may incur higher track costs, but may provide structural cost savings. Refer to Chapter 6 for additional information on design and construction of direct fixation special trackwork. 4.6.8 Noise and Vibration The vehicle traveling over the direct fixation track produces noise and vibration. The impact of this noise and vibration generally becomes significant on alignments through sensitive areas, such as near hospitals. Track design has a significant effect on both noise and wheel squeal, and the designer must consider the wheels, trucks, and the track as one integrated system. Chapter 9 provides guidelines with respect to trackwork design for low noise and vibration and introduces various concepts in noise and vibration control. Trackwork design and eventual track maintenance (or lack thereof) can have a substantial effect upon wayside noise and vibration. Noise and vibration should be considered early in facilities design to provide for special treatments. Cost-effective designs consider the type of vehicle involved, the soft primary suspensions that produce ideal levels of ground vibration above 30 Hz, or the stiff primary suspensions that produce levels that peak at 22 Hz. See Chapter 9 of this Handbook for detailed discussion of these issues. 4.6.9 Direct Fixation Track Communication and Signal Interfaces The light rail transit signaling system may include track circuit signal systems within the direct fixation track zones. Although design of the communication and signal control systems will not

Track Structure Design 4-91 greatly impact direct fixation track design, it can affect specific parts of the design. The prime example of this interrelationship is the need for insulated joints in the running rails to accommodate train control requirements. Such joints are normally required at the extremities of interlockings, at each end of station platforms, within individual turnouts and crossovers, and at other locations to be determined by the train control design. Insulated joints in opposite rails at the limits of the track circuits must be within 4 feet [1.2 meters] of each other to facilitate underground ducting and traction cross bonding. Impedance bonds are often located adjacent to insulated joints at the limits to interlockings and intermediate signals. The installation requirements for these impedance bonds, including consideration of storm water drainage paths, must be coordinated with concrete plinth track structure design. For additional information on transit signal work, refer to Chapter 10. 4.6.10 Overhead Contact System—Traction Power Requirements of the traction power system, including the overhead contact system (OCS) impact the track design at two specific locations: • The catenary pole locations in relation to the track centerline. The catenary poles impact the direct fixation track centerline distance and aerial structure width when they are located between the tracks. Dynamic clearance distances pertinent to the transit vehicle and any other potential users (i.e., track maintenance vehicles) are a design issue that must be jointly considered by the track and catenary designers. • When the running rail is used as a negative return path, its electrical isolation from the ground is essential and the specific resistivity of the electrical insulating products used in the track system is a key design issue. • The electrical continuity of the running rail must also be considered so that the resistance of the entire traction power circuit is as low as possible. Secondary cables (“rail bonds”) are used as jumpers around bolted rail joints and within special trackwork to achieve this goal. Impedance bond boxes at insulated joints and cross bond cabling are included in these design considerations. For additional information on traction power issues, refer to Chapter 11. 4.7 EMBEDDED TRACK DESIGN Embedded track is perhaps the single most distinguishing characteristic of a light rail transit system. Deceptively simple in appearance, it can be quite difficult and expensive to successfully design and construct. Generally, embedded track is required for one of two reasons: • The LRT track is located in a street within a shared traffic lane and must accommodate rubber-tired vehicle traffic.

Track Design Handbook for Light Rail Transit, Second Edition 4-92 • The LRT track is located in an exclusive separated guideway or lane with curbs, but, for reasons of aesthetics or housekeeping, it is deemed inappropriate to use an open trackform such as ballasted or direct fixation. In addition to typical structural design issues that affect any track, embedded track design must also address difficult questions with respect to electrical isolation, acoustic attenuation, and urban design, all in an environment that does not facilitate easy maintenance. The “correct design” may be different for just about every transit system. Even within a particular system, it may be prudent to implement two or more embedded track designs tailored to site-specific circumstances. There are generally four types of embedded track designs: • Concrete slab track structure wherein the rail is embedded in concrete. Within the slab are cross ties to hold the rail to gauge and an elastomeric element (usually rail boot) to provide electrical and vibration isolation. • A concrete slab with formed troughs for each rail. The rails are suspended in the trough and the spaces beside and beneath them are filled with an elastomeric grout that provides both electrical isolation and acoustic attenuation. The slab holds the track to gauge, and there are no distinct rail fastenings. This is sometimes called a “floating rail” design, since there are no rail fasteners of any sort and the rail is held to line and grade solely by the cured elastomeric grout. • A concrete slab with troughs as above, except that the rails are mounted on direct fixation rail fasteners for electrical and acoustic attenuation, and the spaces alongside of the rails are filled with a premolded filler material. • Conventional ballasted track with surface embedment to encase only the ties and rail. This is what the AREMA Manual for Railway Engineering, Chapter 12, calls “paved track.” Many variations of each of the above can be found, usually in response to specific project conditions, with the differences limited only by the ingenuity of the track designers. 4.7.1 Embedded Track Defined Embedded track can be described as a track structure that is completely encased—except for the tops and gauge sides of the rails—within pavement. Portland Cement Concrete is generally preferred as the pavement material; however, other options include brick or blockstone (also known as “cobblestone”) pavements, usually in combination with concrete details. Flangeways can be provided either by using groove rail or by forming a flangeway in the embedment material when tee rail is used. Embedded track is generally the standard for light rail transit routes constructed within public streets, pedestrian/transit malls, or any area where rubber-tired traffic must operate. On several transit systems, both highway grade crossings and tracks constructed in highway medians have used embedded track. “Paved track” is different from embedded track. AREMA’s Manual of Railway Engineering, Chapter 12, Rail Transit, Part 8, defines the two as follows:

Track Structure Design 4-93 • Embedded Track is founded on a concrete slab, similar to non-ballasted track [e.g., direct fixation track] … the paving infill is usually concrete or asphalt, but can also be pavers, paving stones, grass, etc. • Paved Track … is ballasted track of various types (concrete, wood, steel or plastic ties in crushed stone ballast, etc.) covered with either asphalt, concrete or other type of pavement. Experience suggests that paved track, as defined above, generally provides unsatisfactory long- term performance under contemporary light rail transit loadings, particularly if it is also subjected to loadings from heavy highway vehicles such as tractor trailers. Construction and reconstruction of track within urban streets is a lengthy process and can be extremely disruptive to the community. It is therefore recommended that the trackform be robust and expected to have a long service life. A service life of not less than 25 years is suggested, if for no other reason, so that when reconstruction does become necessary, nobody in the community can argue that the tracks were built “only a few years ago and why didn’t they do it right the first time?” (There is something about urban transit that has always inspired irate citizens to write nasty letters to the editor.) Delivering an embedded track design with a long service life will defuse one of the common arguments against rail transit and will also virtually always win out on a life cycle cost basis. Embedded track can be constructed to various designs, depending on the requirements of the system. Some embedded track designs are very rigid while others are quite resilient. Prior to developing an embedded track design, several vehicle/track-related interface issues must be examined and resolved, including vehicle wheel gauge, wheel profile, and truck axle spacing design; the track gauge and rail section; and the ability of the vehicle to negotiate the track in a satisfactory manner. These are addressed in other articles of this chapter. Embedded track is very often located in acoustically sensitive areas, and the designer must also consider noise and vibration mitigation measures as described in Article 4.7.7. 4.7.2 Embedded Rail and Flangeway Criteria To develop embedded track design, the following track components and standards must be specified: • Rail section(s) to be used: groove rail, tee rail, or some combination of both. • Track gauge. • Flangeway width(s) provided in grooved rail or formed flangeway. • The use (or not) of either restraining rail or a groove rail section suitable for use in guarding curves in embedded tracks. Refer to Chapter 5, Article 5.2, for guidance concerning selection of rail sections. See Article 4.2 of this chapter for discussions concerning track gauge and flangeway width. See Article 4.3 of this chapter and Chapter 5, Article 5.3, for discussions of restraining rails in embedded track.

Track Design Handbook for Light Rail Transit, Second Edition 4-94 4.7.2.1 Embedded Rail Details at the Rail Head The rail section and wheel profile used on a transit system must be compatible. Further, the rail installation method must be carefully detailed if the track system is to be functional, have minimal long-term maintenance requirements, and realize the expected rail life. Legacy street railway/tramway systems typically used wheels with relatively narrow tread surface widths and narrow wheel flanges. The chief reason for the narrow tread was to ensure minimal projection of the wheel tread beyond the rail head where it could contact the adjoining pavement, damaging both the wheel and the pavement. Some systems used wheels with tread widths as narrow as 2 inches [50 millimeters] and overall wheel widths of only 3 inches [75 millimeters]. Problems with these wheels, particularly in the vicinity of non-flange-bearing special trackwork design, resulted in newer systems adopting wheels with much wider treads. Wheels with an overall width of 5 ¼ inches [133 millimeters], adopted from railroad standards, are common on new start systems. However, increasing the wheel tread width beyond the rail head introduces an overhang with potential for interference between the outer edge of the wheel tread and the embedment materials. This is particularly the case when European groove rail sections are used, since they all have heads that are only about 60 mm [2.4 inches] wide. To avoid wheel or concrete pavement damage, either the rail head must be raised above the surrounding embedment material or the concrete pavement immediately adjacent to the rail must be depressed, as shown in Figure 4.7.1. Slightly elevating the rail above the pavement is the usual detail shown on drawings; however, it is extremely difficult to actually construct that way. More achievable is finishing the concrete pavement on the field side of the rail at about ¼ inch [6 mm] below the top of rail out to a line roughly 5 inches [12 cm] beyond the gauge line of the rail. This allows for a fair amount of rail wear before wheel contact with the pavement might occur. Other factors must be considered when positioning the rail head with respect to the concrete pavement surface: • In resilient embedded track design, a rail head vertical deflection ranging from 0.06 to 0.16 inches [1.5 to 4 millimeters] must be considered. • Vertical rail head wear of 0.4 inches [10 millimeters] or more must be accommodated. • The wheel tread surface will wear and, depending on the diligence of the LRV wheel maintenance program, can result in a false flange height of 1/8 inch [3 millimeters] or greater. Over the life of the installation, the total required vertical displacement between the rail head and the pavement surface immediately adjacent to the rails could exceed 0.6 inches [15 millimeters]. A 0.6-inch [15-millimeter] or more projection of the rail above the pavement would be excessive for an initial installation. Such a rail projection could hinder snow plowing operations at grade crossings and could be hazardous, especially for bicycles, motorcycles, and pedestrians. It would also be a violation of ADAAG in pedestrian areas. A maximum 0.25-inch [6-mm] projection is recommended for initial installation, which should accommodate the designed resilient vertical rail deflection, some initial vertical rail head wear, and a moderate amount of false flange wheel wear.

Track Structure Design 4-95 False flanges should not be allowed to progress, especially to the -inch [3-millimeter] height, and the track designer should stress that the vehicle system maintenance policies must include a regular wheel truing program. When rail head wear has eliminated approximately half of the projecting ¼-inch [6-millimeter] vertical head clearance, the original projecting dimension can be restored by production surface grinding of the surrounding embedment material. Figure 4.7.1 Embedded rail head details 4.7.2.2 Wheel/Rail Embedment Interference The width of a light rail vehicle wheel is a major design issue. Each design option has certain drawbacks such as the following: • Wide wheels increase the weight (mass) on the unsprung portion of the truck and project beyond the field side of the head of virtually any rail section that might be considered for LRT use. Wide wheels are therefore also susceptible to developing hollow treads and wide false flanges and could require more frequent wheel truing to maintain acceptable tracking through special trackwork.

Track Design Handbook for Light Rail Transit, Second Edition 4-96 • Narrow wheels result in limited tread support at open flangeways and increase the possibility of wide gauge derailments. This typically forces the adoption of either flange-bearing special trackwork or the use of movable point frogs. • Medium wheels partially solve the problems noted above, but still have the problem of undesirable wheel tread protrusion beyond the field side of narrow rail head designs. Medium wheels also provide limited tread support in special trackwork and usually require flange-bearing special trackwork or movable point frogs. As stated in Article 4.7.2.1, embedded track design must consider the surrounding embedment material’s exposure to the overhanging or protruding wheel treads. See Chapter 5 for dimensional information on various popular rail sections, including head width. If wheel tread width exceeds the rail head width of the selected embedded rail, interference between the outer edge of the wheel and the embedding pavement is inevitable as the rail wears vertically. Assuming 115 RE rail, any wheel tread much wider than 2 ¾ inches [70 millimeters] will extend beyond the field side of the rail head. Adding a 1-inch [25.4-millimeter] thick flange to that dimension, plus an allowance for gauge freeplay, limits the total wheel width to about 4 inches [100 millimeters] or less. The conflict is even more acute if European groove rails are used since they typically have heads that are only 56 mm [about 2.2 inches] wide. The former ATEA sought to minimize such problems by having no standard wheel tread more than 3 inches [75 millimeters] wide and making the heads of their standard girder groove and girder guard rail sections that same dimension; however, those rail sections are no longer available. A railway wheel or transit wheel that overhangs the rail head must be clear of the surrounding embedment material, as shown in Figure 4.7.1. Raising the rail head will facilitate future rail grinding and delay the need for grinding the surrounding embedment material to provide clearance for the wheel tread. Embedded track top-of-rail tolerances must be realistic when considering concrete slab placement during track construction. A projection of 1/8 to ¼ inch [3 to 6 millimeters] above the surrounding surface is realistic. Rail positioned lower than 1/8 inch [3 millimeters] above the pavement is not recommended. Trackside appliances such as electrical connection boxes, clean out drainage boxes, drainage grates, and special trackwork housings must be depressed or recessed in the vicinity of the rail head to provide for various wheel tread, rail wear, and rail grinding conditions. As a guideline, depressed notch designs in the covers, sides, and mounting bolts of the track enclosures adjacent to the rail head are recommended. A depth of 5/8 inch [15 millimeters] should provide adequate clearance throughout the life of the rail installation. 4.7.3 Embedded Track Types Chapter 2 documents the types and magnitudes of loads transferred from the vehicle wheel to the rail. The rail must support the vehicle and the resulting loads by absorbing some of the impact and shock and transferring some of the force back into the vehicle via the wheels. The initial impact absorber on the vehicle is the elastomer in the resilient wheel, followed by the primary suspension system (chevron springs), and then the secondary suspension system (air bags).

Track Structure Design 4-97 The initial impact absorber on the track is the rail, specifically the rail head, followed by the fastening or supporting system at the rail base, and then the remaining track structure. The track structure’s degree of resiliency dictates the amount of load distributed to the rail and track structure and the magnitude of force returned to the wheels and vehicle. 4.7.3.1 Non-Resilient Embedded Track Rail supported on a hard base slab, embedded in a solid material such as concrete with no surrounding elastomeric materials, has a high modulus of elasticity and will support the weight of the vehicle and absorb a moderate amount of the wheel impact and shock. A majority of the impact loads will be transferred back into the vehicle via the wheels. Non-resilient rail can be considered as a continuously supported beam with a minor amount of rail base longitudinal surface transfer. Non-resilient track has had mixed success. Eventual spalling of the surrounding embedment and surface failure are common problems. This is especially evident in severe climates where freeze/thaw cycles contribute to track material deterioration. Concrete embedment alone does not provide rail resiliency. It creates a rigid track structure that produces excessive unit stresses below the rail, causing potential concrete deterioration. Such designs are highly dependent on the competency of the concrete immediately adjacent to the rails. Non-resilient embedded track slabs also tend to resonate and can be a significant source of noise. They are also prone to rail corrugation, exacerbating the noise conditions. Field quality control during concrete placement and vibration are very important. Rigid track was usually successful under relatively lightweight trams and streetcars, but it has often failed prematurely under the higher wheel loadings of the current generation of light rail transit vehicles. The size and mass of the base slab, typically a concrete slab 12 to 24 inches [300 to 600 millimeters] thick, tends to dampen some impacts generated by passing vehicles. This results in reduced and usually minor transfers of vibration to surrounding structures. For more information on track slab noise and vibration attenuation, refer to Chapter 9. Several transit systems feature embedded rail suspended in resilient polyurethane materials. This rather simple form of embedment completely encapsulates the rail, holding it resiliently in position to provide electrical isolation and full bonding of the rail and trough to preclude water intrusion. These installations have been successful with no visible defects. Experience has shown that polyurethane has a tendency to harden and lose some of its resiliency over time. This hardening results in surface deterioration from wheel contact, but that wear does not progress to the point where it is detrimental to surrounding structures or otherwise considered faulty by the general public. Like all engineered structures, these installations age and slowly deteriorate to the point where eventual replacement is required. Some products will, by virtue of their compounding, perform better under some conditions than others. The track designer is encouraged to carefully research candidate products and to compare the vendor’s product information against the expected service conditions. Compared to Portland Cement Concrete, bituminous asphaltic embedment materials provide a minor degree of resiliency, but tend to shrink, harden, and crumble with age, leading to excessive interface gaps between the rail and asphalt or roadway concrete. When bituminous asphalt

Track Design Handbook for Light Rail Transit, Second Edition 4-98 hardens, it tends to fracture and break down. The resulting water intrusion will accelerate deterioration of the entire track structure, especially in freeze/thaw climates. As a guideline, although both direct concrete embedment (without any intermediate isolation layer) and bituminous asphalt materials have been used in track paving embedment, they are not recommended for main track use. An elastomeric rail boot or other elastomeric components are available to provide resiliency at the rail surface and potential rail deflection both vertically and horizontally. However, direct embedment of rail into concrete without a rail boot or other separation/isolation can be utilized in very slow speed shop tracks where electrical isolation is not required and vibrations are generally insufficient to initiate cracking of the concrete. Tracks operated at speeds greater than 10 mph [16 km/h] should isolate the rail from concrete with a boot to minimize the chances of concrete deterioration. 4.7.3.2 Resilient Embedded Track Direct fixation transit track and conventional ballasted track are both resilient designs with a proven record of success. This success is due, in no small measure, to their ability to deflect under load, with those deflections being within acceptable operating limits for track gauge and surface. These rail designs are able to distribute loads over a broad area, thereby avoiding— except for the rail-wheel contact—point loading of the track structure that could cause track failure. Resilient track has been successful in ballasted track and direct fixation track installations and has had improved results in embedded track installations. Non-resilient embedded track designs typically fail in excessive loading situations, such as a very sharp curve, where the rigid nature of the embedment materials prevents the rail from distributing loads over a broad enough area thereby overstressing portions of the structure. A key goal in embedded track design is to duplicate the rail deflections and resiliency inherent in ballasted and direct fixation track systems to provide an economical long-term track structure. Rail supported on a resilient base with a moderate modulus of elasticity and embedded on a solid track slab will support the weight of the vehicle and absorb and distribute a greater amount of the wheel impact and shock. Some of the impact load will be transferred back into the vehicle via the wheels. Resilient rail evenly distributes vehicle loads along the rail to the surrounding track structure. The operational frequency ranges developed by each light rail vehicle will determine the design parameters of the resilient track structure design and its components. The guidance of a noise and vibration expert is highly recommended to coordinate the design of the resilient track structure with the light rail vehicle’s proposed primary suspension system and vehicles generally equipped with resilient wheels. The participation of the vehicle design team is obviously required. Resilient wheels attenuate some of the vibration caused by wheel-rail contact. The vehicle’s primary suspension system, although not part of the track design, has a direct bearing on both wayside vibration and reduction of the vibrations entering the carbody, thereby affecting ride quality. However, the design of these vehicle features is focused on mitigating noise and vibration within the light rail vehicle. They do not provide significant attenuation of ground-borne acoustic effects. For additional information on noise and vibration, refer to Chapter 9.

Track Structure Design 4-99 4.7.3.3 Floating Slab Embedded Track Ground-borne noise and vibration are a concern for embedded track sections adjacent to or near facilities that are sensitive to noise and vibration. These include hospitals, auditoriums, recording studios, symphony halls, schools, laboratories, and historic (potentially fragile) buildings—to name a few. Numerous methods for controlling ground-borne noise and vibration exist, including floating slabs, ballast mats, rockwool batts, and soft, highly resilient, direct fixation rail fasteners. The decision to use floating slab design is based on site-specific critical requirements and is often the preferred method to dampen and control the transfer of low-frequency ground-borne noise and vibration in the embedded track. Floating slab design generally consists of two separate concrete structures with resilient isolators positioned between them. The initial base slab is constructed on the subgrade or tunnel invert. The second slab, which includes the track structure, is supported on the resilient isolators and has no direct contact with the base slab. The base slab is usually U-shaped, making the entire structure somewhat similar to the “bathtub” concept. The resilient supporting isolators between the U-shaped base slab and the sectional track slab can take several forms. Most common, particularly in older installations, are large diameter elastomer “hockey pucks” or “donuts” that are sized, spaced, and formed to provide the desired spring rate and acoustic attenuation. Some installations have substituted ballast mat sheets and rockwool batts for the donuts. Steel springs have been used in some installations, although extreme caution must be taken when placing spring steel in a potentially damp and corrosive environment where inspection and housekeeping is difficult. In all cases, resilient isolators must also be placed between the sides of the sectional track sections and the vertical walls of the base slab to both limit lateral track movement and provide acoustic isolation. The wall isolators can either be individual elastomer blocks, continuous elastomer sheeting, or ballast mats extending up the base slab wall. The track slab can take many forms depending on the requirements. For straight and moderately curved track in tunnels, the usual floating slab design is segments, each supporting four direct fixation rail fasteners—two under each rail. Such slabs are often called “double ties.” For embedded track, the slabs are typically 20 to 30 feet [6.1 to 10.4 meters] in length. When the floating slabs are used in embedded track, the exposed joint between the track slab and the base slab must be well-sealed to limit water intrusion and accumulation of surface contaminants in the voids around the base isolators, which will degrade the system’s performance. In all cases, drainage of the void area in the bathtub area beneath the track slab is critical. The design should provide manholes for periodic visual inspection and the flushing out of the void area beneath the slab. The design of undertrack vibration isolation systems should be based on site-specific rail features, vibration radiation, and the distance to surrounding structures and is best undertaken by a noise and vibration expert experienced in not only dampening and isolation but also the basics of railway track design, construction, and maintenance. For additional information on noise and vibration, refer to Chapter 9.

Track Design Handbook for Light Rail Transit, Second Edition 4-100 4.7.3.4 Proprietary Resilient Embedded Rail Designs The design of a noiseless and vibration-proof track system has been and will likely continue to be an elusive task. Experimental, usually proprietary, design concepts to curtail noise and dampen vibrations are continuously emerging from many sources. This Handbook is necessarily limited as to detailing the various proposed designs; should the reader be interested in researching the state-of-the-art concepts, papers, articles, advertising, and other information is available through transit industry convocations, magazines, and professional journals. The reader/designer is cautioned that, while new products for railway systems appear frequently, many do not survive in the marketplace. For example, the first edition of this Handbook included at this point an illustration of a then-new embedded rail system that appeared to offer some promise as a niche solution for vibration attenuation. However, nothing has been heard of that product since then. Manufacturers of unusual proprietary products often times go out of business after a few years, making future procurement of spare parts nearly impossible. 4.7.4 Concrete Slab Track Structure Concrete slab embedded track designs have evolved and consist of various styles; however, the three most successful and hence favored designs are the following: • A continuous single-pour concrete slab encapsulating the entire track structure, leveling beams (steel channel ties) and the rail encased in a premolded elastomeric rail boot securely connected to the leveling beams. See Figure 4.7.2. • A continuous single-pour concrete slab with two formed rail pockets or troughs for the installation of the rails (see Figure 4.7.3). Stray current protection is provided at the rail within the trough area by the placement of a polyurethane insulating filler. • A two-pour concrete slab with cold joint between the two pours located at the base of rail. Stray current protection is provided either at the rail or within the formed trough area as described above. The initial concrete track slab width can be designed to accommodate both single-track and double-track installations. As a guideline, the preferred design for ease of installation is two single-track concrete slab pours. For installations that are not in mixed traffic, the two track slabs would be separated by an open area called “the devil strip.” The devil strip is generally filled with non-reinforced concrete to complete the track slab installation. An alternate method, if track centers are not too wide, is an expansion joint at the centerline of both tracks. When one or both tracks are in mixed traffic, the roadway pavement design will usually govern the configuration of the areas outside of the track slab. See Chapter 12 for additional guidance concerning track slab design in mixed traffic areas. The required accuracy of the track alignment and the finished top- of-rail concrete surface should control the construction limits staging and methods of embedded track construction. See Chapter 13 for additional guidance on embedded track construction methods. Embedded floating track slab designs are rare and very special. There are typical designs, as described above, for embedded rail within the floating slab. Nevertheless, a project-specific design to meet the vibration mitigation requirements and also match the site limitations must be

Track Structure Design 4-101 Figure 4.7.2 Embedded track on leveling beams

Track Design Handbook for Light Rail Transit, Second Edition 4-102 Figure 4.7.3 Concrete slab with individual rail troughs designed jointly by track designers, structural engineers, noise and vibration experts, and the project’s vehicle engineers. Similar joint efforts are required to adapt generic concepts for floating slabs supporting special trackwork to each specific project. 4.7.4.1 Embedded Rail Installation The methods for installation, positioning, and retention of the rail depend on the specific design criteria selected. Whatever system is employed, it must be able to rigidly hold the rails to proper rail cant inclination as well as control track gauge and alignment. 4.7.4.1.1 Top-Down Construction—Rail Support and Gauge Restraint Booted rail on leveling beams is the most popular embedded track design at the time of the writing of this Handbook. The design consists of a single-pour concrete embedment of the entire track structure, requiring a method for accurately positioning the skeleton track in both the vertical and horizontal positions. The rail encapsulated in a pre-formed rail boot is securely fastened to the leveling beams, forming the skeleton track system that will be embedded in the concrete track slab, as shown in Figure 4.7.2. Some sort of gauge control and adjustment is needed during top-down embedded track construction, especially in sharply curved track. Even if the rail is pre-curved, some adjustments to gauge will always be necessary because it is not possible to pre-curve rail to precise radii

Track Structure Design 4-103 within the tolerances required for track gauge. It must be possible to adjust the track gauge both in and out at any point along the rail in both tangent track and curves. 4.7.4.1.1.1 Gauge Rods. Gauge rods attached to or through the web of the rail was a traditional method of achieving track gauge adjustment; however, this method greatly complicates achieving electrical isolation. Some LRT projects in the 1980s and 1990s used gauge rods, but they have generally fallen out of favor. 4.7.4.1.1.2 Steel Ties/Leveling Beams. The prevailing standard is leveling beams or steel ties beneath the rail. Both use structural members (typically steel) with a small cross-sectional area to hold the rails to gauge and alignment. Leveling beams incorporate a simple screw jack arrangement on the ends of the tie to permit rapid adjustment of the rails to precise profile. Some projects place these on both ends of each tie. Others place the leveling screw on only one end, but alternate the orientation of the tie so there effectively is a screw jack on every other tie on each side of the track. Plain ties without the integral screw jack require separate chairs to elevate the rail to profile grade and may not be as stable in supporting the skeletonized track against unintended movement. Rail cant, if required, can be achieved by either bending the ends of the ties or by incorporating tapered shims in the rail fastening system. Steel ties/leveling beams have proven to be a satisfactory method for setting and adjusting track gauge, and, since they are compatible with the rail boot, it is far easier to achieve electrical and acoustic isolation than when using gauge rods. On many projects, the cross ties in embedded track will be spaced very widely—as much as 10 feet [3 meters] in some cases. This is because, once the embedding reinforced concrete is poured and cured, it is what holds the rails to gauge, not the ties. The ties are there merely to hold the rails to the proper line and gauge until such time as the encasing concrete can take over that duty. Depending on the corrosion control measures used on the project, the leveling beams/steel ties may be epoxy coated to match the reinforcing steel. Note that the system used to support the ties above the prepared subgrade may provide a direct path to ground from the steel tie. In the case of the leveling beams, it may be appropriate to place an HDPE pad beneath the base plate on the bottom of the leveling screws. Plastic ties, manufactured specifically for embedded track, have been used on many projects. Being dielectric, they have a distinct advantage with respect to electrical isolation. However, they may not have sufficient strength for gauge adjustment, particularly if curved rails need to be pushed out relative to each other. Hence, plastic ties may be best if they are used only on tangent track and steel ties (or leveling beams) are used in curved track. The use of steel ties and gauge bars in embedded track sections tends to produce a surface crack in rigid pavements directly above or near the member. To control surface deterioration, a scored crack control slot is recommended. This may not be specifically necessary in installations where the pavement surface consists of brick or other individual pavers, although pavers have been known to crack at such locations.

Track Design Handbook for Light Rail Transit, Second Edition 4-104 4.7.4.1.1.3 Rail Fastenings for Leveling Beams. The booted rail must be firmly held to the ties or leveling beams. There are two principal methods: • Rigid rail clips fastened to the tie using a threaded fastening. If made of steel, these clips are often used with an isolating pad (similar to those used on concrete cross ties) between the nose of the rail clip and the booted base of the rail. This pad protects the rail boot from abrasion. At least one manufacturer offers a plastic rail clip that eliminates the need for the isolating pad. • Elastic rail clips, made of spring steel, and generally identical to those used on concrete cross ties, including the isolating pads as noted above. These spring clips are often covered with a plastic cap (both resembling and commonly called a baseball “batters helmet”) so the subsequent concrete pour does not flow around and encase the spring clip. Some points to consider about the rail clips discussed above: • Elastic rail clips in embedded track are virtually always installed by a laborer using a sledge hammer. A missed hammer swing could impact and damage the rail boot. The damage may not be visually apparent without close scrutiny, and it is unlikely that the laborer will bring the incident to the attention of somebody in charge who could perform a closer examination. An undetected tear in the rail boot could become a stray current leak. • Plastic rail clips are not likely to have sufficient strength to resist rail rollover and uplift forces. One design of plastic clip, when subjected to the vertical uplift test commonly applied to rail fastening systems, failed at a fraction of the design load. If plastic clips are used, the design of the track slab, particularly the configuration of the reinforcing steel, should be sufficiently robust to keep the rail in position without reliance on the rail clips. • One frequently voiced concern in embedded track design is that it should be possible to replace the rail without disturbing the underlying slab. This interest leads many designers to choose elastic rail clips as they believe that product will make it easier to perform the rail renewal. However, consideration should be given to whether, after some reasonable rail service life, the spring clips will actually still be in a serviceable condition. Even with the plastic cap, there is a good possibility that accumulated moisture around the clip may have initiated some corrosion, leading to a “frozen” installation and possible failure of the clip. A failed spring clip would be concealed by the pavement structure and hence undetectable. Even if the clip has not failed, exfoliated rust may have locked the clip into the shoulders of the steel tie making it impossible to remove the clip without damaging the shoulders. • Rigid rail clips can also be covered with a plastic cap (or some sort of mastic coating) to protect the threaded elements from corrosion and may be more likely to be in serviceable condition after a lengthy period. • As a point of reference, legacy streetcar systems would often get 30, 40, or more years of service out of embedded tracks. Typically, the only areas where earlier rail renewal

Track Structure Design 4-105 might be required was at passenger stops, where braking and tractive efforts (including the use of magnetic track brakes) would wear the top of rail much faster than on plain running track. Notably, such service life was achieved with rails made of steel a little more than half as hard as the steel now available. Designers should consider whether it is likely that the rail at a given location might require renewal within the service life of the track slab before including extraordinary measures to facilitate a rail replacement that might never need to occur. 4.7.4.1.2 Floating Rail Installation Floating rail installation, as illustrated in Figure 4.7.3, relies on the embedment materials to secure and retain the rail in position without any mechanical connections between the rail and the track slab. In this configuration, the rail installation is a two-step, top-down process as illustrated in Figure 4.7.4. First, the rail is either positioned within the trough (left side of Figure 4.7.4) or on the initial concrete base slab (right side of Figure 4.7.4) using temporary jigs. Next, sufficient trough or base embedment material (typically polyurethane or an elastomeric grout) is placed to completely encapsulate the base of rail, thereby locking the rail in its final position. The temporary jigs are then removed, and a second application of trough fill material generally encapsulates the remaining rail to top of rail. However, note that some polyurethanes, once cured, do not bond well to additional layers of the same product, especially if appreciable time has elapsed since the first pour. Manufacturer’s recommendations must be followed closely. If groove rail is used, no special surface finishing is required. If tee rail is employed, either a flangeway can be formed on the gauge side of the rail head or the embedment material can be deliberately left low, subject to meeting ADAAG requirements for flangeway within areas where pedestrians might legitimately need to cross the track. Regardless of rail section, the surface of the embedment material must be left low on the field side of the rail to provide for false flange relief and future rail wear, as described previously. Figure 4.7.4 Floating rail embedment—base material installation 4.7.4.1.3 Alignment Control in Top-Down Construction Meeting construction tolerances for either skeleton track or floating rail installations depends on the contractor’s ability to rigidly hold the assembled track or rails, respectively, in proper

Track Design Handbook for Light Rail Transit, Second Edition 4-106 alignment during the initial embedment material pour. Once set, the rail position cannot be adjusted to meet construction tolerances or future maintenance needs. Irregularities in the rail alignment due to either accidental displacement of the rail during placement of the embedding material, including thermal movement during construction, can only be fixed by removal and replacement. Maintaining the alignment during the embedment pours can be especially difficult in curved track. The contract specifications should require the contractor to submit a detailed work plan, including quality control measures, that demonstrates understanding of the issues and the steps that will be necessary for meeting the specified track tolerances. It is good practice to require the acceptable construction of a demonstration section of track before the contractor’s method is approved for production work. 4.7.4.1.4 Bottom-Up Embedded Rail Installation Bottom-up construction typically involves constructing an underlying track slab and then anchoring the rails to it after the slab has cured. Rail fastening installations use mechanical rail base connections to secure the rail in position. The installation may consist of the following methods: • Core drilling and epoxy grouting the fastening insulated anchor inserts or bolts to the initial concrete slab, as shown in Concrete Slab “A” of Figure 4.7.5. • Using cast-in-place insulated anchor inserts during the initial track slab concrete construction, as shown in Concrete Slab “B” of Figure 4.7.5. Such designs require limited horizontal and vertical track alignment adjustment prior to embedment. This is provided by the leveling nuts and slotted holes in the rail base plate. Slotted plate holes may provide for horizontal adjustment and additional shims for vertical adjustment. In these rail installation fastening designs, the stud bolts in Concrete Slab “A” of Figure 4.7.5 are insulated from the plate by insulating washers and thimbles. In Concrete Slab “B” of Figure 4.7.5, the plate is similarly insulated from anchorage assemblies and the base concrete. In both cases, the trough fill material is an insulating elastomeric grout. Neither design, as shown, provides any acoustic attenuation and would have a very high track modulus. The addition of a rail boot would provide some attenuation and soften the track; however, designers are cautioned that details that require a rail boot to flex over the edge of a plate may lead to boot failure and a possible stray current leak. Rail fastening embedded track designs must consider the ability of the rail to distribute lateral loads to the rail fasteners. If the rails are rigidly secured at centers of approximately 35 to 40 inches [900 to 1000 mm] and the surrounding embedment materials are more flexible, the track will have hard spots that will cause the rail to wear abnormally and may possibly induce corrugations. Elastomer pads are essential if the encasement material is resilient. If rail is contained within a rail boot, a separate pad is unnecessary, and the embedment material can be a non-resilient concrete or cementitious grout. If a super-resilient installation is desired, a separate base pad can be designed to establish the spring rate. Direct fixation rail fasteners may be used to secure the rail to the base slab. The fasteners provide resiliency in all directions as well as electrical isolation. Anchor plates may also be used. The benefits of using anchor plates in embedded track are

Track Structure Design 4-107 • Rigid control of rail position during two-pour initial installations. • Anchor plates can be reused during future rail changeout to control rail position. • Track can be used in partially completed installations to either confirm track installation or maintain revenue service. Figure 4.7.5 Rail fastening installations 4.7.4.2 Stray Current Protection Requirements The most effective mitigation strategy for prevention of stray current corrosion is to insulate both rails at their surfaces. The track structure requires that an electrical barrier (such as a rail boot or encapsulation of the rail in an insulating resilient polyurethane material) be provided at the rail. Refer to Chapter 8 for additional details on the theories of stray current.

Track Design Handbook for Light Rail Transit, Second Edition 4-108 Principal measures to minimize traction current leakage are the following: • The use of a rail section providing electrical resistance not exceeding 0.0092ohms/1000 feet at 20 degrees C. • The use of continuous welded rail providing superior traction power return over conventional electrically bonded jointed track. • Insulating either individual rails or the entire track structure from the earth. • Insulating embedded switch machines and any other embedded track system appliances from the earth. So as to provide for the safety of maintenance personnel, it may be necessary to provide features for temporary grounding of switch machines. • Continuous welding of the steel reinforcement (if present) in the supporting base slab, to act as a stray current collector, and electrical drains to carry intercepted current back to the traction power substation. Several transit agencies construct embedded track with no reinforcing steel in the underlying slabs. This has the advantages of eliminating the need to weld or electrically bond the steel and eliminating it as an attractive alternative path for stray currents. Alternatively, the reinforcing steel can be epoxy coated, which will reduce or eliminate its tendency to attract stray current from the rails. • Cross bonding of rails with cables installed between the rails to maintain equal potentials for all embedded rails. • Rail bond jumpers at unavoidable mechanical rail connections, such as rail joints, especially within the special trackwork installations. Appliances that are bolted to the rail without intervening insulation should also be electrically bonded to the rails to minimize corrosion. Key details concerning the above measures that affect the track structure design are the following: • Type of insulation to be installed, whether it is located at the rail face or around the entire periphery of the track structure as in the bathtub concept. • Type of insulation to be provided at/around the earth boxes that contain switch machines and other signal and traction power system attachments to the track. • Provisions such as earth boxes for traction power cross bond cables and conduits between rails on each track and between rails on different tracks. • Ductwork that must be provided in the embedment materials. • Provision for rail bond jumpers exothermically welded to the rail on either side of a bolted joint or completely around non-welded special trackwork components prior to embedding the track. Prior to installation of the embedded track structure, a corrosion survey should be undertaken to establish the existing baseline stray current levels. Periodic monitoring should be performed after installation of embedded track to detect current leakage and to control or improve insulation performance. Stray current protection design can include one or more of the following concepts:

Track Structure Design 4-109 Encasing the rail in an insulating elastomeric (rubber) boot and thereby totally encapsulating the surface except for the rail head and gauge face. Joints between contiguous segments of rail boot must be sealed with overlapping “cuffs” that are glued to the boot. Constructing the entire embedded track slab within a trough (commonly called a “bathtub”) with an insulating liner between the track slab and the inner surface of the trough. This method was commonly used for special trackwork, but is now less popular due to the development of easier methods of insulating oddly shaped special trackwork components. Coating of the rail surface (except the head and gauge face) with an insulating dielectric epoxy, such as coal tar. This is sometimes used for insulating special trackwork areas within a bathtub. In lieu of field-applied coatings, several vendors have come up with methods of encapsulating stick rails and special trackwork in an relatively thick elastomeric membrane. These components can then be assembled at the jobsite, including field application of insulating products to seal the joints against stray current leakage. Embedding the rail and filling the entire trough with an insulating dielectric polyurethane or other suitable insulating material. Insulating any anchor assemblies that penetrate the insulated rail trough and pass into the underlying concrete track slab. This insulation is typically a dielectric coating of the anchor assembly where it is in contact with the track slab. Additional information on corrosion control is included in Chapter 8 of this Handbook. 4.7.4.3 Rail Insulating Materials Materials for electrically isolating the rail from the surrounding pavement range from the very elaborate and expensive to the simple and moderately priced. Materials include manufactured, extruded rail boot and modular insulating block modules (see Figure 4.7.6), cast-in-place resilient polyurethane components, concrete fills of various compositions, and an asphaltic bituminous mortar. Costs will vary by the application and the project and are but one consideration for evaluation when selecting the most appropriate design for a given project. Embedment designs for resilient track that utilize the general track structure, as described above, have incorporated the following materials to retain and allow for designated rail deflections with varying success. Figure 4.7.6 Extruded elastomer trough and rail boot for tee rail

Track Design Handbook for Light Rail Transit, Second Edition 4-110 4.7.4.3.1 Extruded Elastomeric Rail Boot and Trough Components Rail boot has proven to be a highly satisfactory rail base support material that provides minimal rail deflection. Extruded elastomeric rail boot sections are designed to fit and enclose the entire rail section, exposing only the head and flangeway in groove rail and the head and gauge face surface in tee rail. The boot design consists of an insulating elastomeric (rubber) configuration shaped to fit the rail, providing internal air voids for displacement of incompressible elastomer, which allows deflection that makes the elastomer resilient. Resilience is often measured as spring rate. Rail boot is a single-component insulating material used as described above to generate a resilient embedded track system. Rail boot provides a form fit to the periphery of the rail; however, because the boot is necessarily flexible, there isn’t a watertight seal between the rail and the boot. It is therefore virtually certain that storm water and any contamination it might carry will seep inside of the rail boot. To minimize this, it is essential that the constructor be directed to devise a means of keeping the boot tight up against the rail during concrete placement. The use of “duct tape” to temporarily hold the boot in place is the usual method, although some contractors have devised other techniques. Loose boot could result in localized corrosion within the boot that might produce sufficient exfoliated rust to damage the boot. Many designers choose to provide a path by which moisture within the boot can drain by removing a portion of the boot at the base of rail at each track drain. Arguably, this is unnecessary since moisture inside of the boot would not, due to the lack of sufficient oxygen, initiate significant corrosion of the rail. Removal of the boot at the track drain might also provide a stray current path to ground, resulting in corrosion of the rail where it is suspended over the drain. Opinions vary widely on this issue, and the designer needs to closely consider the ramifications of draining versus not draining the boot and whether the configuration of the track drains facilitates inspection and maintenance of the track isolation system. A possible solution would be to remove the boot through the track drain but to coat the bare rail with an epoxy material that overlaps the boot on either side of the drain. Refer to Figure 4.7.8 for a suggested track drain detail. Rail boot designs are currently available for both common North American tee rail sections and popular groove rail sections. Boots are also available for tee rail and bolted restraining rail assemblies including both vertically mounted restraining rail and strap guard. Experience suggests that the rubber molding industry is reasonably accommodating in producing relatively small quantities of rail boot for non-standard shapes. As an insulating material, extruded elastomer rail boot has proven to exceed the required bulk resistivity of 1012 ohm-cm that is needed to be effective. In addition to rail boot, other designs of extruded modular insulating materials have been developed to fit the rail web and base of rail contour. These products, which are popular in Europe, typically include • An insulating strip that lays beneath and grips the edges of the base of rail. • Insulating blocks that are laid above the rail base and alongside of the web and head of the rail.

Track Structure Design 4-111 Using extruded insulation requires the two-pour method for base slab installation, including installation of the rail prior to placing the surrounding extruded component sections. Finally, the top concrete surface is then placed beyond the gauge and field sides of the extrusion. Providing insulating protection to the total rail surface, including any portion of the rail base not in contact with extruded sections, is an important requirement. Extruded sections are available in separate parts that encase the entire rail as shown on the left of Figure 4.7.6. These designs require a specific concrete base installation sequence to provide complete support under the base of rail. As an insulating material, extruded elastomer has proven to meet the required bulk resistivity of 1012 ohm-cm that is needed to be effective. 4.7.4.3.2 Resilient Polyurethane Polyurethane components can be used as trough fillers. Resilient polyurethane has proven to be an ideal rail base support material that provides a minimum of rail deflection. Altering the consistency of the polyurethane compound to adjust its durometer hardness can control the actual amount of deflection. Since polyurethane grout is typically rather expensive, compressible filler materials such as cork particles or shredded rubber or sand are often added to minimize costs. Compressible fillers such as cork also provide internal shape factors to polyurethane elastomeric grouts and thereby reduce the track modulus. Designers must carefully consider the effects such fillers might have on efficacy, durability, and service life. Elastomeric polyurethane is an effective stray current protection barrier that binds well to both cleaned rail surfaces and concrete trough surfaces. It is, however, expensive, both for material procurement and the labor associated with mixing and installation. To reduce the volume of polyurethane required, premolded rail filler blocks shaped to fit the web of the rails can be used, as shown in Figure 4.7.7. The embedment design must consider rail base deflections. Embedment materials for both the rail head and web areas should be resilient in nature to allow for the rail vertical and horizontal deflection. Solid or non-resilient encasement materials surrounding the rail will negate the resilient characteristics of the polyurethane and could lead to premature failure of the non-resilient materials. Figure 4.7.7 Polyurethane trough filler with web blocks As an insulating material, polyurethane has proven to meet the required bulk resistivity of 1012 ohm-cm. Polyurethanes are a difficult and expensive material for in-track construction. Some urethanes are highly susceptible to chemical reaction with moisture in the air, the fine sand additive for bulk, and surface dampness during application. Their chemical characteristics make it essential that

Track Design Handbook for Light Rail Transit, Second Edition 4-112 mixing, handling, and application be carefully undertaken only by qualified contractors with product representatives present for initial installations to train the installation crew. Polyurethanes can be very difficult to install in tracks with any significant gradient as they flow to form a level surface when in liquid form. It is often necessary to pour the polyurethane in short segments between temporary “dams.” Even then, the finished surface may be irregular and somewhat unsightly. 4.7.4.3.3 Elastomer Pads for Rail Base Elastomer pads are a satisfactory rail base support material that provides a controlled amount of rail deflection depending on the spring rate of the elastomer and its specific durometer hardness. Natural rubber elastomer pads mixed with proper quantities of carbon black and wax have exhibited satisfactory performance and long life. Although water seepage typically will not damage the elastomer pads, proper drainage of the rail trough should improve performance, provide insurance that the expected life cycle will be realized, and increase the effectiveness of the pads as a stray current deterrent. The embedded track design must consider rail base deflections with matching resilient rail web and head embedment materials to allow for rail movement. Solid or non-resilient embedment materials surrounding the rail will defeat the elastomer pad’s resiliency and lead to premature failure of the non-resilient materials. As an insulating agent, either synthetic elastomer compounds or natural rubber have met the required bulk resistivity of 1012 ohm-cm. 4.7.4.3.4 Elastomeric Fastenings (Direct Fixation Fasteners) To duplicate successful, open, direct fixation track design with acceptable rail deflections, some elaborate embedded track designs have incorporated direct fixation concepts. Bonded direct fixation fasteners and component plate and elastomer pad fastenings have been housed beneath protective covers allowing the spring clip flexure. The trough fill is a matching spring rate material to match the fastener characteristics. This is not a recommended installation. Obvious seepage around the rail and into the open cavities would likely carry with it contaminants such as road salts, which, in the presence of moisture, initiate corrosion, particularly at the spring steel rail clips. The cavity would likely always be damp, leading to failure of the clips due to corrosion, exacerbated by localized stray currents. 4.7.4.3.5 Concrete and Bituminous Asphalt Trough Fillers Concrete and cementitious grout components are non-insulating and should not be used as trough fillers in embedded track construction, except for non-insulated shop track installations. Bituminous materials, such as conventional asphalt mixes, will generally crumble over time and are not recommended as a suitable trough filler. Rubberized asphalt mixes are appreciably cheaper than elastomeric grouts and polyurethane, but have shown mixed results in service, particularly under heavy vehicular traffic loading. 4.7.4.4 Embedded Track Drainage In all but the driest climates, the success of most embedded track designs will depend directly on the efficacy of the embedded track’s drainage systems. This includes not only systems for intercepting surface runoff, but also methods for draining both water and the contaminants it carries that seep into the rail cavity zone. Experience has shown that surface water will seep and accumulate in the rail area, particularly around the rail base and web. In cold weather climates,

Track Structure Design 4-113 this accumulated moisture can freeze and damage both the rail embedment materials and the electrical isolation systems. Less severe damage can occur in the absence of freezing temperatures. Deterioration of the surrounding pavement structure is possible, eventually leading to failure of the embedded track system, with a high probability of unacceptable levels of stray traction power current. Sealing the interface between the booted rail and the adjoining concrete embedment material is virtually impossible. Many track designers therefore assert that drainage of the internal and external surfaces of the booted rail embedment system is of the utmost importance, especially at track profile alignment sags. Similarly, unsealed construction cold joints and expansion joints will allow water entry. Lateral expansion joints abutting the rail boot will allow draining at the rail boot surface. Regardless of how well the surface sealants are designed and installed, seepage will eventually occur and possibly lead to deterioration or disintegration of the fill components, particularly in climates susceptible to freeze/thaw cycles. To prevent this, the embedment booted rail system can be designed with a reliable permanent drainage system, as shown in Figure 4.7.8. Figure 4.7.8 Typical embedded track drain chase

Track Design Handbook for Light Rail Transit, Second Edition 4-114 Polyurethane trough fills, on the other hand, provide an excellent bonding to the rail and the surrounding concrete, sealing off water seepage. 4.7.4.4.1 Surface Drainage Embedded track installations complicate pavement surface drainage because the exposed rail head and flangeways intercept and redirect lateral storm water runoff. Normal sheet flow to the curbline does not occur, especially if the track slab is transversely flat/level within the roadway cross section. The track slab may be designed to direct the water to the centerline of each track to control runoff to the nearest transverse track drain. In addition, if the roadway pavement is crowned in the conventional manner, the pavement cross slope results in the track being out of cross level in tangents and perhaps even negatively superelevated in curves. While undesirable, these conditions can be accepted within limits. See Chapter 12 for additional commentary on these conditions. For additional information on track surface and cross level, refer to Chapter 3. Whenever possible, the profile and cross section of the road should be modified to conform to the optimum track profile and cross section. This often requires that the roadway geometry be compromised to accommodate rail elevations, curb and gutter elevations, and sidewalk grades. Not all projects can afford to substantially alter the street outside of the trackway. The optimal and affordable situation typically requires compromises to the design criteria for both the LRT and the roadway owner. The surface runoff entering the flangeways should be minimized by design, and trackway road surfaces should ideally slope away from the embedded rail locations. The pavement surfaces on the field side of the rails should ideally slope away from the rails and toward the curb. Conventional flangeways will inevitably intercept and carry storm water runoff. This runoff must eventually be intercepted and drained away from the track. Transverse lateral drainage the full width of the track slab should always be provided at the low points of vertical curves and immediately up-grade of both embedded special trackwork and transitions between embedded track and any open track design. Drains immediately up-grade of embedded special trackwork installations are particularly important so as to intercept storm water runoff and the debris it carries before it can enter the switches and interfere with switch operation. Additional drainage points should be provided periodically along straight track grade sections so that runoff, debris, sand, or other material can be carried away and the flangeway kept relatively clear. Typically these track drains will be no more than 1000 feet [300 meters] apart, depending on track gradients. Draining the flangeways is of greater importance in snowbelt climates where accumulated water might freeze in the flangeway and cause a derailment. These intermediate drains can be either the full width of the track slab or smaller units that drain only the flangeway, depending on local conditions. Drains in embedded track areas are typically transverse drains or drainage chases perpendicular to the rails and a minimum of 12 inches [300 mm] wide. They consist of a grate-covered chamber that is connected to a nearby storm sewer system. These drains should not be located in pedestrian areas, but the grate should be bicycle safe even if no such traffic is expected in the track area. Note that, as of this writing, there are no specific ADAAG requirements concerning drain grates within accessible paths, likely because such features should be mutually exclusive.

Track Structure Design 4-115 The design of the rail through the drainage chase opening should consist of the exposed rail supported on each side of the chase wherein the rail acts as a suspended beam. The bottom of the flangeways must have openings wide enough to ensure that they will not become clogged with leaves or other debris. This is achieved relatively easily with tee rail construction. If groove rail is employed, it is common to machine a slot in the bottom of the flangeway, leaving the tram intact. However, such slots typically cannot be much more than 7/8 to 1 1/8 inches [19 to 28 millimeters] wide, and they therefore frequently become clogged with leaves, paper, and other medium-sized street debris. For this reason, these drains function more reliably if used with tee rail. Figure 4.7.8 illustrates a typical embedded track drainage chase. Where clogging is likely and the tram of the groove rail is not required as a restraining rail, an alternative design is to cut away a portion of the tram through the drainage chase area. This must be done with machine shop equipment and precision so that the rail is not structurally impaired. Flame cutting of any sort, including plasma cutters, should not be used unless extensive grinding is subsequently performed to remove the heat-affected metal. Regardless of the cutting method, smooth radii should be provided at cut corners and edges. Inside corners should preferably have a radius of about 2 inches [50 mm], and external corner radii should be no less than ¾ inch [20 mm]. All edges should be rounded to a radius of ¼ inch [6 mm] or more, both to eliminate stress risers and decrease the chance of debris catching on the edge. The flangeways should be flared to protect the cut ends of the tram from being struck by wheels. If flame cutting was used, testing to detect the presence of untempered martensite should be done, and additional grinding done if it is found. Where embedded track does not need to routinely accommodate either pedestrian or rubber-tired traffic, it is possible to simplify trackway surface drainage by using tee rails, setting the pavement surface between the rails at what would normally be the flangeway depth, and sloping the pavement down to a gutter along the centerline of track. Ordinary storm drains are placed along that gutter to intercept flow and take it to a storm sewer system. Since tee rails are used, draining the flangeway becomes a non-issue. This configuration can minimize stray current issues since accumulation of street debris adjacent to the non-insulated head of the tee rail, where it could bridge the top edge of the rail boot, is no longer a concern. This design has been successfully employed on several LRT line extensions. Figure 4.7.9 illustrates this configuration on the Portland LRT system. This detail can still accommodate the occasional operation of rubber-tired traffic such as rail system maintenance trucks and public safety vehicles such as police, fire, and emergency medical services. However, the drivers of such vehicles need to be trained to not attempt any sudden turns when a tire is in the depressed gauge area of the track. Proactive measures should be taken to exclude general traffic from these areas, especially bicycle and motorcycle traffic, since the operators of such vehicles, not understanding the risks, could lose control when attempting to cross a rail at too high a speed.

Track Design Handbook for Light Rail Transit, Second Edition 4-116 Figure 4.7.9 Depressed pavement without flangeways 4.7.5 Ballasted Track Structure with Embedment Early 20th-century embedded track designs for urban streetcars and trams was typically ordinary ballasted track with timber cross ties that was subsequently embedded to the top of rail with some type of conventional paving material. Blockstone or brick was very common, and concrete and asphalt were also used. In general, such systems do not provide satisfactory long-term service under today’s LRV and highway traffic loadings. In addition, it is difficult to ensure long-term electrical isolation with such designs. While it is possible to wrap the rail in a rail boot, the constant flexure of the boot over the edges of the rail fastener will likely lead to a boot failure and stray current leakage. Therefore, designs such as the one illustrated in Figure 4.7.10 are not recommended. The blockstone or brick surfaces on the embedded track systems of legacy streetcar systems were often very durable, sometimes even lasting for decades after the trolley cars quit running. Their nostalgic appearance frequently causes urban planners to desire such surfaces for modern LRT tracks. Extreme caution should be exercised if the decision is made to use such details. To begin with, the materials and methods that are commonly used to build brick or cobblestone streets today are different and arguably inferior to the methods used in the early to mid-20th Century. Paving brick is no longer made to the same ASTM C7 specification, a design that featured lugs to keep the individual bricks slightly separated and a re-pressed and fire-glazed surface on all six sides. The pavers were often times underlain with a plain, unreinforced concrete slab with an intermediate setting bed of bituminous asphalt rather than sand. The joints between the blocks or brick were often filled not with mortar but rather hot tar. Streets built in this fashion were extremely durable. The success of these brick and blockstone surfaces is likely due in part to their articulated structure and resultant ability to adjust to vehicle loads and thermally

Track Structure Design 4-117 induced movements. The key to this ability was the use of hot tar to seal the joints between the pavers, thereby excluding most moisture. The tar was also self-healing so that any cracks or separations of one block from another would be resealed on the next hot weather day. It is perhaps notable that some tramway systems in Europe are imitating these old modular pavements with manufactured asphaltic blocks, but still using bituminous sealer at the joints as shown in Figure 4.7.11. Figure 4.7.10 Ballasted track structure with embedment A drawback of brick or blockstone paving materials is creating and maintaining an appropriate flangeway under the crush of heavy rubber-tired traffic while still maintaining electrical isolation of the rail. The use of groove rail makes special details unnecessary. Modular paving blocks can still be used with tee rail, although it becomes necessary to take other steps to provide a uniform flangeway. Figure 4.7.12 illustrates a concept for utilizing pavers as the roadway surface while still providing a formed flangeway in concrete. Results that would be similar in appearance could be achieved by using a stamped concrete finish that resembles brick paving. However, the durability of the faux brick finish may be inferior to authentic pavers. Readers are also cautioned that the pigments used to tint stamped concrete finishes often include compounds such as iron oxide and could exacerbate any tendency for stray currents to leak across the pavement surface. Modular concrete panels, similar to concrete grade crossing panels, are popular as a pavement material for tramway tracks in some European countries, particularly in the former Soviet bloc. These panels are precast and can be made for specific locations in tangent or curved track and within special trackwork units. However, photographs of such installations on the Internet suggest that the quality of both the installations and the material itself can vary widely. Measures must still be taken to exclude water intrusion beneath the panels and to deal with such moisture and contaminants as may make it through the joint sealants.

Track Design Handbook for Light Rail Transit, Second Edition 4-118 Figure 4.7.11 Bituminous pavers with sealed joints Figure 4.7.12 Use of brick or stone pavers with embedded tee rail

Track Structure Design 4-119 4.7.6 Embedded Special Trackwork The embedded special trackwork portion of any transit system will require special treatment and quite possibly a different design concept from the main line embedded track design. In contemporary light rail transit systems, embedded special trackwork generally consists of turnouts for entry onto other track(s) or pairs of turnouts grouped to act as single crossovers for alternate track operations. Operating requirements and restricted right-of-way conditions may dictate the installation of a double crossover consisting of four turnouts and a central crossing diamond. An extensive embedded track transit system could utilize complex embedded special trackwork arrangements beyond simple single and double crossovers. For example, embedded special trackwork layouts are often used in yards and shops for streetcar systems and can be extremely complex. For additional information on embedded special trackwork design arrangements, refer to Chapter 6. The size, configuration, and complexity of the components; the requirements for stray current protection; and the need to secure the components to the trackbed dictate special trackwork embedment design. The special trackwork designer could contemplate two general types of installations: • The first option is stray current protection at the surfaces of each turnout component (switch housing, switch machine earth box, frog, and turnout electrical rail boxes with cable conduit) and the rail face. • The second option, often used to simplify the insulation of the overall special trackwork installation, is the bathtub design. The first option is generally difficult due to the requirement to insulate the irregular surfaces of a wide array of turnout components. Proprietary systems have been developed to encapsulate individual special trackwork components in a shop environment prior to assembly of the layout in the field. The joints or welds between those components are then encapsulated during final field assembly, typically using a compatible elastomeric grout material. Special measures are necessary to accommodate any electrical cables that might need to pass through the electrical isolation barriers. Separate measures might be necessary to provide acoustic attenuation if the encapsulation material does not provide sufficient resiliency. The bathtub design positions the stray current protection completely clear of the individual special trackwork components. The special trackwork is constructed within a reinforced concrete box (the “bathtub”) that has been completely lined with a dielectric insulating membrane. The only penetrations through this perimeter insulating barrier are openings for the rails, electrical conduits for train control or traction power purposes, and storm water drain conduits from the switch machine case. Each of those penetrations is insulated separately. This significantly simplifies the special trackwork insulating installation compared to Option 1 above. Figure 4.7.13 shows one possible configuration for a bathtub. Depending on the requirements of the train control system, the bathtub design may not completely eliminate requirements for insulating rails in the special trackwork layout from each

Track Design Handbook for Light Rail Transit, Second Edition 4-120 other. In addition, additional measures may be necessary to provide acoustic attenuation, both to mitigate possible ground-borne vibrations and to prevent the pavement that encases the rails from resonating and amplifying vibrations within the track structure. Figure 4.7.13 Special trackwork—embedded “bathtub” design

Track Structure Design 4-121 Embedded special trackwork will also require the use of special leveling beams or plates to support the various track elements. These must be designed to develop uniform rail deflections matching the adjacent track system. For additional information on embedded special trackwork component design, refer to Chapter 6. 4.7.7 Noise and Vibration The interface of vehicle wheel to rail is another contributor to noise that is virtually impossible to eliminate. Vehicle wheel loads are transmitted from the wheel/rail interface to the track structure. Unlike ballasted or direct fixation track, with load distribution to the ties or fasteners, very nearly all embedded track designs use a concrete slab and continuous elastomeric rail support system to distribute the load throughout the surface of the rail base. The resilient elastomeric rail support system in embedded track (typically either rail boot or a trough filled with polyurethane) dampens the rail, reducing rail vibration and rail-radiated noise. The characteristics of the resilient elastomer system control the degree of vibration and deflection. A softer elastomer provides a lower spring rate in the rail support, leading to reduced vibration in the rail. The spring rate is used in determining the track modulus or track stiffness and the amount of vertical deflection in the rail. The track elastomer, in conjunction with the vehicle primary suspension system, affects the vehicle/rail interface—specifically, track performance, noise, and vibration in the immediate rail area. Noise and vibration control should be considered in the vehicle truck design, particularly with respect to the use of resilient wheels and the details of the primary suspension system. The primary suspension is located between the journal and the truck frame. The primary suspension characteristics (chevron design) are dependent on the elastomeric spring elements, number of layers or total deflection, and their angular formation. The elastomeric spring of the suspension reduces noise by acting as a vibration isolator. It also acts as a barrier to the transmission of structure-borne noise. See Chapters 2 and 9 for additional discussion concerning vehicle truck design and noise. In selecting the suspension characteristics of the extruded elastomer, elastomeric base pad, or the rail boot elastomer used to support the rail, vehicle parameters such as normal weight and crush loads must be considered. Each light rail vehicle, with different truck suspensions, wheelbases, and weights may require a different track dynamic suspension system. The advice of a noise and vibration expert in this endeavor is recommended, as stated in Chapter 9 of this Handbook. Because it is inherently stiffer than open trackforms and possibly also because its extremely uniform support facilitates the wave transmission of vibrations, embedded track can become corrugated more quickly than ballasted or direct fixation track. This propensity makes it even more important to provide an extremely smooth wheel/rail interface and to maintain it in that condition. Precision rail grinding to produce an extremely smooth rail surface free of imperfections such as mill scale from both the original rolling and the heat treatment process is strongly recommended.

Track Design Handbook for Light Rail Transit, Second Edition 4-122 4.7.8 Transit Signal Work Transit signal requirements in embedded track sections differ from the general design standards for ballasted and direct fixation track. Embedded track within city streets may share the right-of- way with automobiles, trucks, and buses both at intersections and along the track. Depending on the conditions, the train control systems could be very rudimentary (“line-of-sight” operation) or quite sophisticated. Signal equipment “earth boxes” to accommodate switch machines and appurtenances such as loop detectors for train-to-wayside communications and electrical conduits to connect these items to wayside controls may need to be pre-installed in embedded track areas prior to placement of pavement. The track slab design will need to accommodate these items as well as drainage systems to keep such embedments dry. The design of the embedded track must anticipate and accommodate these systems. The input of signal designers who are experienced in train control systems for embedded track is essential. It may be necessary for the track designer to encourage project management to accelerate that part of the signal design so that signal system accommodations can be incorporated in the track design. On many projects, train control requirements have not been identified until after track construction commenced, requiring a major demolition effort to correct the oversight. Similar to signals, traction power requirements in the way of earth boxes with conduit connections for power and track circuits may be needed. 4.7.9 Traction Power Traction power requirements in embedded track sections differ from the standards for ballasted or direct fixation track. The need to keep the rail electrically isolated from ground is a major part of overall embedded track design. Unlike ballasted and direct fixation track standards, where the rail can be relatively easily insulated from the ground at either the base of rail or within the fastening system, embedded track requires that the entire rail surface except top of rail and gauge face be insulated. This requirement contributes to the challenge of designing embedded rails that provide an insulated, resilient, and durable track system using off-the-shelf materials. Embedded ductwork in the form of earth boxes and conduits within the track structure provides access for power cables and cross bonds to achieve equalization in the rails. The design of embedded track must contemplate the complete requirements for traction power with the input of experienced traction power design experts well before track design is completed. As in the case of train control systems mentioned above, omissions of traction power embedments can be extremely difficult and expensive to correct. For additional information on stray current control and traction power, refer to Chapters 8 and 11, respectively. 4.7.10 Turf Track European light rail transit systems have been leaders in blending the light rail transit guideway into the surrounding landscape and streetscape. Toward that end, many European cities have included turf track (also known as “grass track”) or trackway landscaping in construction of new

Track Structure Design 4-123 LRT lines and reconstruction of older tram routes. Landscaped track was developed for various reasons, including • Reducing the visual impact of the track system compared to either ballasted or direct fixation track. • Reducing the noise from the rail operation due to the soft turf’s ability to absorb under-vehicle noise rather than reflecting it to the environs. Landscaped track has proven to reduce noise by 6 to 8 dBA. Turf track has become a popular concept for light rail routes that must pass through or near environmentally sensitive areas such as parklands, university or business campuses, residential neighborhoods, or other areas where the use of either open trackforms or embedded track is undesired. The earliest turf track installations were nothing more than ordinary ballasted track where the ballast section had become extremely fouled, and natural vegetation had subsequently gotten out of hand. In time, the track structure became completely concealed by soil and turf, with the exception of the tops of the rails. The appearance was visually pleasing, at least to lay persons with no responsibility for track inspection or maintenance, with the result that maintaining the track area as if it were a lawn became a de facto standard. The problems with these primitive turf track installations include the following: • A complete lack of any sort of electrical isolation. Since nearly the entire rail is in contact with moist soil, traction power currents can stray from the rail at virtually any location. The base and web of the rail can lose a significant percentage of their cross-sectional area in a relatively short time, long before head wear might justify rail renewal. Rail base corrosion would typically be worst at the rail fastenings (e.g., track spikes) with the resulting loss of rail stability against gauge widening and rail rollover. • Drainage of the trackway must deal with the conflicting goals of keeping the turf sufficiently moist to sustain growth and keeping the subgrade sufficiently dry to maintain a stable base to support the track and the live loads of the rail vehicles. • Fouling of the ballast section by migrated fines from the soils supporting the turf, further exacerbating drainage issues. • Sustaining growth of the turf during dry weather. In North America, the streetcar system in New Orleans has always been the largest user of deliberately created turf track. The track in the St. Charles Avenue streetcar line was totally reconstructed in the early 1990s, and the traditional ballasted turf track was restored with only minor modifications to details used for the better part of a century before. In the early 2000s, New Orleans restored the Canal Streetcar line with extensive stretches of turf track, but utilized a modified embedded track system instead of ballasted track beneath the turf. Turf track has been adopted (or at least considered) in several other cities with light rail or streetcar systems. The streetcar line in Kenosha, Wisconsin, is the largest grass track installation in North America

Track Design Handbook for Light Rail Transit, Second Edition 4-124 outside of New Orleans. Kenosha placed turf over fairly conventional concrete cross tie ballasted track. Issues with turf track that should be addressed during the design process include the following: • Electrical isolation must be maintained to protect the rail against stray currents. • Trackway maintenance activities include maintenance of a lawn. • Adjustments to track alignment, either horizontally or vertically, require removal and subsequent restoration of the turf. The deliberate maintenance of water within the trackway, so as to support the turf growth, makes it all that more likely that track surfacing might be required. • Vegetation should be kept away from the rail running surface where it can lubricate the wheel/rail interface and create problems with traction and braking. • Keeping pedestrians out of the trackway can become nearly impossible since the lay person’s perception may be that the area is a public park. (The turf track in New Orleans is commonly used as jogging path, especially in the vicinity of universities.) LRV operators must be especially alert for persons trespassing on the tracks, and this could have a direct effect on the maximum practical train speeds. • In cold weather climates, snow removal is complicated, since ordinary snowplow trucks cannot be used without likely damage to the turf. • The type of grass to be used in the track area should be carefully selected by a landscaping professional based on factors such as the local climate, the depth of the soil available, how well drained the soil will be, the presence or absence of a sprinkler system, the amount of foot traffic that might be expected, and the maintenance capacity of the owner. A slow-growth grass that reaches a maximum height of approximately 1 ½ inches [40 mm] is preferred to minimize mowing requirements. Several variations of turf track have been employed in projects in Europe and North America. Some of the concepts include • Filling the area on the gauge and field sides of the rails with modular concrete matrices that can support limited rubber-tired traffic but still permit grass to grow in the track area. • Configuring the grass track generally to resemble plinthed direct fixation track albeit with the space below the elevation of the tops of the plinths filled with soil and turf. The rails and rail fastenings sit above the elevation of the turf, greatly simplifying issues of electrical isolation and also making the turf track look much less like a public space. However, the appearance is generally unsatisfactory to persons for whom aesthetics are the overriding issue. Since there are no set “standards” for turf track, many turf track designs similar to embedded track or partially embedded track have evolved. Figure 4.7.14 shows a sample turf track installation based on specific assumed conditions. It consists of concrete plinths or beams running parallel under the rail to support the track. Each rail is installed in a continuous elastomer rail boot. The booted, insulated rails are connected to conventional periodic leveling beams to

Track Structure Design 4-125 hold gauge throughout the installation. The base of rail is not connected to the concrete plinth. The rail boot is secured in position and protected by a continuous concrete divider for separation of earth from rail boot. The surrounding concrete is fashioned similar to conventional embedded track, allowing for wheel passage and rail deflection.. Figure 4.7.14 Turf track 4.8 LRT TRACK ON BRIDGES With the occasional exception of some urban streetcar routes, virtually all LRT lines eventually cross some sort of bridge. Chapter 7 extensively addresses matters of LRT track on bridges from the perspective of the structural engineers. 4.9 REFERENCES [1] Albert S. Richey, Electric Railway Handbook, Second Edition, McGraw-Hill Book Company, Inc., 1924 (Reprinted by the Association of Railway Museums). [2] William W. Hay, Railroad Engineering, Second Edition, A Wiley - Interscience Publication ISBN 0-471-36400-2. [3] Wilson, Ihrig & Associates, Inc., “Theoretical Analysis of Embedded Track Vibration Radiation, San Francisco Municipal Railway,” Technical Memorandum to Iron Horse Engineering Co., 7/17/97. [4] AREA Manual of Railway Engineering (1984), Chapter 22. (NOTE: Chapter 22 does not exist in the current AREMA Manual of Railway Engineering. Readers who wish to consult this reference must secure a copy of the old loose leaf Manual.)

Track Design Handbook for Light Rail Transit, Second Edition 621-4 [5] A.N. Talbot, Reports of the Special Committee on Stresses in Railroad Track, Proceedings of the American Railway Engineering Association, First Progress Report, Vol. 19, 1918, pp. 873–1062; ibid., Second Progress Report, Vol. 21, 1920, pp. 645–814; ibid., Third Progress Report, Vol. 24, 1923, pp. 297–450; ibid., Fourth Progress Report, Vol. 26, 1925, pp. 1084– 1246; ibid., Fifth Progress Report, Vol. 31, 1930, pp. 65–336. [6] D. Read and D. Li, TCRP Research Results Digest 79: Design of Track Transitions, Transportation Research Board of the National Academies, Washington, D.C., October 2006. [7] X. Shu and N. Wilson, TCRP Research Results Digest 82, Use of Guard/Girder/Restraining Rails, Transportation Research Board of the National Academies, Washington, D.C., 2007. [8] X. Shu and N. Wilson, TCRP Report 71: Track-Related Research—Volume 7: Guidelines for Guard/Restraining Rail Installation, Transportation Research Board of the National Academies, Washington, D.C., 2010. [9] Mark O’Hara, “Testing Girder Rail on the MBTA,” Interface—The Journal of Rail/Wheel Interaction, October 2007. [10] Arnold D. Kerr, Fundamentals of Railway Track Engineering, First Edition, Simmons Boardman Books, Inc., 2003. [11] S. Timoshenko and B.F. Langer, Stresses in Railroad Track, Paper APM-54-26, Transactions of the American Society of Mechanical Engineers, Vol. 54, p. 277, 1932.

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TRB’s Transit Cooperative Research Program (TCRP) Report 155: Track Design Handbook for Light Rail Transit, Second Edition provides guidelines and descriptions for the design of various common types of light rail transit (LRT) track.

The track structure types include ballasted track, direct fixation (“ballastless”) track, and embedded track.

The report considers the characteristics and interfaces of vehicle wheels and rail, tracks and wheel gauges, rail sections, alignments, speeds, and track moduli.

The report includes chapters on vehicles, alignment, track structures, track components, special track work, aerial structures/bridges, corrosion control, noise and vibration, signals, traction power, and the integration of LRT track into urban streets.

A PowerPoint presentation describing the entire project is available online.

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