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2-i Chapter 2âLight Rail Transit Vehicles Table of Contents 2.1 INTRODUCTION 2-1Â 2.1.1 State-of-the-Art for Light Rail Vehicles 2-1Â 2.1.2 Vehicle/Trackway Interface 2-2Â 2.2 LIGHT RAIL VEHICLE DESIGN CHARACTERISTICS 2-3Â 2.2.1 Introduction 2-3Â 2.2.2 Vehicle Design 2-4Â 18.104.22.168 Unidirectional/Bi-Directional 2-4Â 22.214.171.124 Non-Articulated/Articulated 2-5Â 126.96.36.199 High-Floor/Low-Floor LRVs 2-7Â 188.8.131.52.1 Introduction 2-7Â 184.108.40.206.2 Low-Floor CarsâGeneral 2-8Â 220.127.116.11.3 Low-Floor Car Truck Design 2-8Â 18.104.22.168 Carbody Strength, Crashworthiness, and Mass 2-9Â 22.214.171.124.1 Introduction 2-9Â 126.96.36.199.2 Crash Energy Management 2-10Â 188.8.131.52.3 LRV Bumpers 2-11Â 184.108.40.206.4 Vehicle Mass 2-11Â 2.3 VEHICLE CLEARANCES 2-14Â 2.3.1 Vehicle Clearance Envelopes 2-14Â 2.3.2 Vehicle Static Outline 2-15Â 220.127.116.11 Vehicle Length 2-16Â 18.104.22.168 Distance between Truck Centers 2-16Â 22.214.171.124 Distance between End Truck and Anticlimber or Bumper 2-16Â 126.96.36.199 Carbody Width 2-17Â 188.8.131.52 Carbody End Taper 2-17Â 184.108.40.206 Other Static Clearance Factors 2-18Â 2.3.3 Vehicle Dynamic Envelope/Outline 2-19Â 220.127.116.11 Vehicle Components Related to Vehicle Dynamic Envelope 2-22Â 18.104.22.168 Track Components Related to Vehicle Dynamic Envelope 2-22Â 22.214.171.124 Vehicle Clearance to Wayside Obstructions and Other Tracks 2-22Â 126.96.36.199 Platform Clearances 2-23Â 188.8.131.52 Pantograph Height Positions 2-23Â 2.4 VEHICLE-TRACK GEOMETRY 2-24Â 2.4.1 Horizontal CurvatureâMinimum Turning Radius of Vehicle 2-25Â 2.4.2 Vertical CurvatureâMinimum Sag and Crest Curves 2-25Â 2.4.3 Combination Conditions of Horizontal and Vertical Curvature 2-25Â 2.4.4 Vertical AlignmentâMaximum Grades 2-26Â 2.4.5 Maximum Allowable Track Twist 2-27Â 2.4.6 Light Rail Vehicle Ride Quality 2-29Â 184.108.40.206 Vehicle Natural Frequency as a Factor in Ride Comfort 2-29Â 220.127.116.11 Track Geometrics as a Factor in Ride Comfort 2-29Â
Track Design Handbook for Light Rail Transit, Second Edition 2-ii 2.5 VEHICLE STRUCTURAL LOADS 2-30Â 2.5.1 Static Vertical Loads 2-30Â 2.5.2 Wheel Loading Tolerance (Car Level) 2-30Â 2.5.3 Wheel Loading at Maximum Stationary Superelevation 2-30Â 2.5.4 Unsprung Mass 2-30Â 2.5.5 Truck Design 2-31Â 18.104.22.168 Motorized Trucks 2-31Â 22.214.171.124 Non-Motorized (Trailer) Trucks 2-34Â 126.96.36.199 Load Leveling 2-35Â 188.8.131.52 Inboard versus Outboard Bearing Trucks 2-36Â 2.5.6 Vehicle DynamicsâPropulsion and Braking Forces 2-37Â 184.108.40.206 Tolerances 2-37Â 220.127.116.11 Maximum Train Size 2-37Â 18.104.22.168 Load Weight 2-38Â 22.214.171.124 Sanding 2-38Â 126.96.36.199 Vehicle Procurement Documents 2-38Â 188.8.131.52 Braking Forces 2-38Â 2.5.7 Dynamic Vertical 2-39Â 184.108.40.206 Primary Suspension 2-39Â 220.127.116.11.1 Spring Rate 2-39Â 18.104.22.168.2 Damping 2-39Â 22.214.171.124 Secondary Suspension 2-39Â 126.96.36.199.1 Damping 2-39Â 188.8.131.52.2 Yaw Friction 2-39Â 184.108.40.206 Maximum Operating Speed 2-40Â 220.127.116.11 Car Natural Frequency 2-40Â 2.6 TRACK GAUGE, WHEEL GAUGE, AND WHEEL CONTOURS 2-40Â 2.6.1 Track Gauge 2-41Â 2.6.2 Vehicle Wheel Gauge 2-41Â 2.6.3 Wheel Profiles 2-43Â 18.104.22.168 AAR-1B Wheel Contour 2-43Â 22.214.171.124 Transit Wheel Design and Selection 2-45Â 126.96.36.199.1 Tread Conicity 2-46Â 188.8.131.52.2 Tread Width 2-46Â 184.108.40.206.3 Flange Face Angle 2-46Â 220.127.116.11.4 Flange/Tread Radius 2-47Â 18.104.22.168.5 Flange Back Angle/Radius 2-47Â 22.214.171.124.6 Flange Height 2-47Â 126.96.36.199.7 Flange Thickness 2-48Â 188.8.131.52.8 Flange Tip Shape 2-48Â 184.108.40.206.9 Wheel Diameter 2-48Â 220.127.116.11 Independently Rotating Wheels (IRWs) 2-48Â 18.104.22.168 Miscellaneous Considerations for Wheel Contours 2-49Â 22.214.171.124.1 Historic Streetcars 2-49Â 126.96.36.199.2 Shared Trackage with Freight Railroad 2-49Â 188.8.131.52 Average Worn Wheel Conditions 2-50Â
Light Rail Transit Vehicles 2-iii 2.6.4 Maintenance of the Wheel/Rail Interface 2-51Â 2.6.5 Matching Wheel and Rail Profiles 2-51Â 2.6.6 Wheel Tread Widths and Flangeways at Frogs 2-53Â 2.7 RESILIENT WHEELS 2-53Â 2.8 ON-BOARD VEHICLE WHEEL/RAIL LUBRICATION 2-55Â 2.9 VEHICLES AND STATIONSâADA REQUIREMENTS 2-56Â 2.10 REFERENCES 2-57Â List of Figures Figure 2.3.1 Three-section 70% low-floor LRV in an 82-foot [25 meter] radius curve 2-18Â Figure 2.3.2 Typical LRV dynamic envelope 2-21Â Figure 2.5.1 Kinki Sharyo power truck for 70% LRV 2-32Â Figure 2.5.2 Siemens power truck for a Combino 100% low-floor narrow gauge LRV 2-33Â Figure 2.5.3 Bombardier Flexity Outlook power truck for 100% low-floor LRV 2-33Â Figure 2.5.4 Kinki Sharyo trailer truck for 70% low-floor LRV 2-34Â Figure 2.5.5 Kinki Sharyo cranked axle for low-floor LRV trailer truck 2-35Â Figure 2.6.1 Candidate initial LRV wheel profile 2-45Â Figure 2.6.2 Compromise wheel for Karlsruhe tram-train 2-50Â Figure 2.6.3 Wheel-rail interface 2-52Â Figure 2.7.1 Bo84 wheels used by NJ Transit 2-55Â List of Tables Table 2.2.1 Relative mass of 100% vs. 70% low-floor LRVs 2-12Â Table 2.2.2 Light rail vehicle characteristics matrix (2010 data) 2-13Â
1-2 CHAPTER 2âLIGHT RAIL TRANSIT VEHICLES 2.1 INTRODUCTION The light rail transit vehicle (âlight rail vehicleâ or âLRVâ for short) is arguably the most publically prominent feature of any LRT system. Everything about the remainder of the LRT systemâs infrastructure, facilities, and systemsâincluding the trackâis designed to make certain the LRVs can fulfill their function of transporting passengers in an efficient and expedient manner. However, LRVs come in a wide variety of designs, and it is essential to understand what the vehicle is before designing the track upon which it will run. 2.1.1 State-of-the-Art for Light Rail Vehicles Major advancements have been made in LRV design since publication of the first edition of the Track Design Handbook for Light Rail Transit. These include but are not limited to the following: â¢ The near total adoption of low-floor and partial low-floor LRVs for virtually all new start projects and also for modernization of other existing light rail systems. Because of this, nearly all new vehicles have one or more trucks that have independently rotating wheels (IRWs) instead of conventional solid axles, adding significantly to the challenges in track design. â¢ Incorporation of crash energy management (CEM) principles in the design of vehicle carbodies. This has the benefit of not only increasing safety in collisions but also significantly reducing both overall vehicle weight and the loads applied by the wheels to the rails. This also reduces power consumption; a study for New Jersey Transit (NJT) concluded that a weight reduction per car of one metric tonne [about 1.1 short tons] can save approximately 24 million kWh of energy over a 30-year life cycle for a fleet of 100 cars, each operating 40,000 miles per year.,  â¢ Improved propulsion system, reducing weight, increasing performance and reliability, and reducing maintenance costs. â¢ Improved AC traction motor/parallel gear units of compact design that are resiliently mounted on the truck frame. â¢ New designs of resilient wheels that are both easier to install and reduce the unsprung mass to that of the steel tire, thus reducing high frequency shock and vibration of both truck and track components. â¢ Adoption of LRVs with multiple (more than two) carbody sections by many transit agencies. Advantages include - Increased vehicle capacity - Reduced vehicle weight per passenger - Reduced number of main propulsion components â¢ Production of light rail vehicles very specifically intended for operation in public streets. These include not only âstreetcarsâ that are somewhat smaller than the previous generation of LRVs but also incorporation of carbody design principles, such as enclosed front bumpers,
Track Design Handbook for Light Rail Transit, Second Edition 2-2 that make even larger LRVs more suitable for operation in areas with large volumes of pedestrians and motor vehicles. â¢ Articulated streetcar vehicles, with the trucks semi-rigidly attached to the carbody rather than swiveling relative to the carbody. Somewhat common overseas since the 1980s, these vehicles have now appeared in North America. â¢ Self-propelled Diesel Mechanical Unit (DMU) passenger railcars are now being operated in several North American cities. While these are not âlight rail vehiclesâ as that term is defined in Chapter 1, they have many similar characteristics. Therefore much of this Handbook is applicable to systems using DMU vehicles. Other changes in light rail vehicle design are occurring, and the list above could be obsolete in a very short time. For example, as of 2011, at least one manufacturer is actively marketing a streetcar-sized LRV for North American use that has âoff-wireâ operating capability. Such vehicles can operate for limited distances without an overhead catenary system by drawing power from an on-board energy storage unit (typically a battery). Off-wire capable vehicles seem very likely to become commonplace as the technology matures. 2.1.2 Vehicle/Trackway Interface As vehicle technology continues to evolve, so does the complexity of the interface between the vehicles and the track. Even more than was the case when the first edition of this Handbook was published, there are few hard and fast rules about the relationships between vehicles and track on light rail transit systems. In spite of this lack of design consistency, there are several key vehicle-to-track and trackway parameters that the track designer must consider during design of light rail systems. These include â¢ Vehicle Weight (both empty and with full passenger load) â¢ Clearances - Required track-to-platform location tolerances to meet ADA requirements - Required clearance between cars on adjacent tracks considering car dynamics - Required route clearances (wayside, tunnel, bridge) considering car dynamics â¢ Wheel Dimensions - Wheel diameter, which can be very small in the case of low-floor vehicles and is virtually always smaller than that used on freight railroad equipment. Smaller wheel diameters produce higher contact stresses than larger wheel diameters, with resulting implications regarding rail corrugation and wear on both wheels and rail - Wheel profile or contour, including the wheel tread width, which must be compatible with the rail section(s) selected, particularly in the case of special trackwork - Wheel gauge, to ensure compatibility with the track gauge, including tolerances - Wheel back-to-back gauge that is compatible with flangeway dimensions and special trackwork check gauges â¢ Longitudinal Vehicle Forces on the Track - Maximum acceleration and associated tractive forces - Maximum/emergency deceleration from a combination of friction brakes, dynamic braking and electromagnetic track brakes, including the automatic application of sand
Light Rail Transit Vehicles 2-3 â¢ Lateral Vehicle Forces on the Track - Maximum lateral forces resulting from all speed and curvature combinations â¢ Dynamic Vehicle Forces on the Track - Impact of car and truck natural frequencies - Impact of wheel flats or damaged wheels It is essential that the track designer, the vehicle designer, and the designers of systems such as signals, catenary, etc., coordinate and cooperate to achieve compatibility between the LRT system components under all operating conditions. These interactions can be facilitated by generating a comprehensive design criteria manual for any new LRT system and keeping it updated with âas-builtâ information as the project is developed, constructed, and operated. It is generally inadvisable to design a new light rail line around the characteristics of only one make and model of light rail vehicle since doing so may limit choices for subsequent vehicle procurements as the system expands and matures. A transit system guideway may remain unchanged for a century or more, during which time it would not be unusual for three or more cycles of vehicle procurements to occur. Instead, it is recommended to consider a universe of candidate LRVs from several manufacturers and develop a fictitious âcompositeâ LRV that incorporates the most restrictive characteristics of several cars, e.g., the longest, the widest, the one with the largest minimum radius capability, etc. In this fashion, the transit agency will not be forever restricted to using only one particular make and model of LRV. It also minimizes situations where parts of the track alignment are at the absolute minimum or maximum capabilities of the vehicle, a condition that is highly discouraged in any event. When new vehicles are procured for an existing transit line, the vehicle must be specified to operate on the existing track unless a concurrent rehabilitation and upgrading of the old guideway is proposed. When an existing transit line is extended, the track standards for the extension must accommodate both the old rolling stock and any new vehicles that might be procured. 2.2 LIGHT RAIL VEHICLE DESIGN CHARACTERISTICS 2.2.1 Introduction Light rail vehicles are built in a variety of designs and dimensions. In almost all cases, they are capable of being operated in coupled trains. Modern LRVs are generally much larger and heavier than their streetcar predecessors and can have axle loads just as large as, or even larger than, so-called "heavy rail" transit vehicles. Notably, the modern streetcars used in one U.S. city actually have slightly higher axle loadings than the light rail vehicles also used there. Light rail vehicles vary in the following design characteristics: â¢ Unidirectional versus bi-directional â¢ Non-articulated versus articulated and, for the latter, the location(s) and configuration of the articulation joints â¢ 100% high-floor versus partial low-floor (typically 70% or less) versus 100% low-floor â¢ Overall size (width, length, and height) â¢ Truck and axle positions â¢ Weight and weight distribution
Track Design Handbook for Light Rail Transit, Second Edition 2-4 â¢ Suspension characteristics â¢ Performance (acceleration, speed, and braking) â¢ Wheel diameter and wheel contour â¢ Wheel gauge These characteristics must be considered in the design of both the vehicle and the track structure. 2.2.2 Vehicle Design 184.108.40.206 Unidirectional/Bi-Directional Nearly all of the legacy streetcar systems in North America that survived up through the 1960s used unidirectional vehicles, most often the Presidents Conference Committee (PCC) streetcar. Such âsingle-endâ cars had operatorâs controls in the forward end, doors on the right side, and a single trolley pole current collector at the rear. At the end of the line, cars negotiated a turning loop and ran to the opposite terminal. Because these vehicles could negotiate curves with centerline radii as small as 35 feet [10.7 meters], the amount of real estate needed for a turning loop was relatively small, usually only a single urban building lot. Transit companies typically found that the expense of buying properties and building loops was small compared to the savings associated with not having to maintain duplicate sets of control equipment in âdouble-endâ trolley cars. Current designs of high-capacity light rail vehicles have much larger minimum radius limitations and the amount of real estate that is required to construct a turning loop is much greater. Accordingly, while a few European light rail lines continue to use single-end, single-sided vehicles that require turning loops, most contemporary LRVs have control cabs in both ends and doors on both sides. These cars can advantageously reverse direction anywhere that a suitable crossover track or pocket track can be provided. This arrangement is usually more economical than the turning loop in terms of real estate required and has become the norm for most modern light rail transit systems. Crossovers and pocket track arrangements can often be sited within the confines of an ordinary double-track right-of-way and do not require the supplemental property acquisition needed for turning loops. The following are some of the factors that should be considered when evaluating single-end versus double-end light rail vehicles: â¢ Systems with stub-end terminals at either one or both ends of the line or at any intermediate turnback location will require bi-directional vehicles. â¢ Bi-directional vehicles with two operating cabs and doors on both sides of the vehicle will cost more than a single-end LRV with only one cab and doors on only one side. â¢ For slow speed movements in a yard or under an emergency situation, many single-end LRVs have a âback-up controllerâ in the rear of the car, often hidden behind a panel or under a seat. â¢ Unless equipped with doors on both sides, single-end LRVs require that all station platforms be located on the same side of the tracks. Having doors on both sides of the vehicle provides the capability of having stations on either or both sides of the track, regardless of whether the vehicle has one operating cab or two.
Light Rail Transit Vehicles 2-5 â¢ Single-end vehicles that have doors on both sides can be coupled back-to-back resulting in a double-end train. â¢ The choice of single-end versus double-end vehicles may have an impact on how yard and shop facilities are laid out. This in turn will affect the real estate requirements for that facility and hence its location. The yard location in turn may have a direct effect on the system operating plan. â¢ Double-end vehicles typically have more uniform wear of the wheels since the leading axle on each truck changes at the stub-end terminals. Single-end vehicles often develop thin wheel flanges on the leading axle of each truck while the flanges on the trailing axles incur relatively little gauge face wear. This directly affects the frequency and cost of wheel truing and ultimately wheel replacement. â¢ From a civil engineering perspective, stub-end terminals are less costly compared with the loops because, as noted above, of the land costs and other local space restrictions. Trackwork costs for a stub-end terminal versus a loop could be similar or greater depending on the configuration and amount of special trackwork associated with any terminal station, passing tracks, or storage tracks. Train control system costs are nearly certain to be greater for a stub-end terminal than for a loop terminal. â¢ Stub-end terminals have construction and maintenance costs associated with special trackwork and train control systems that differ from those of loop tracks. The designer must evaluate options based on life cycle costs. â¢ Dwell times for a loop terminal are appreciably less than those for a stub-end terminal, which can be advantageous at terminals with extremely close operating headways. â¢ If double-end cars are selected, it is still possible to have loops at some terminals should local conditions make that choice advantageous. â¢ Loop tracks are more likely to be sources of noise than stub-end terminals, possibly impacting both the wayside community and patrons alike. The crossover track movements associated with a stub-end terminal are more likely to be a source of ground- borne vibration, particularly if a double or âscissorsâ crossover is used. â¢ Loop tracks at an intermediate turnback point will require a crossing diamond, which is more likely to be a source of noise and vibration than the ordinary frogs in the crossover tracks associated with a center pocket track. â¢ If there is a reasonable probability that a line might be extended beyond some initial terminal location, a stub-end track arrangementâand hence double-ended vehiclesâ would usually be the logical choice. â¢ Stub-end tracks provide greater flexibility for vehicle storage during off-peak hours. 220.127.116.11 Non-Articulated/Articulated The earliest electric streetcars in the 1880s were four-wheeled single truck vehicles. Streetcar ridership quickly outgrew the capacity limitations of such vehicles, and eight-wheeled double truck streetcars were common by 1900. Often, these larger cars would pull a trailer car for even more capacity. The first articulated streetcars appeared in the United States about the time of World War I, often by splicing together two older single truck cars, and later as three-truck vehicles
Track Design Handbook for Light Rail Transit, Second Edition 2-6 functionally very similar to high-floor, articulated LRVs of today. The objective of this evolution in vehicle design was to maximize not only passenger capacity but also the number of passengers carried per operating employee since labor costs, then as now, were a high percentage of the cost of transit operation. That trend has continued up through the present with the result that multiple-section light rail vehicles have reached unprecedented lengths. Today, with the exception of legacy and heritage streetcar operations and three light rail systems that bought new rolling stock in the 1980s, all new and modernized North American light rail systems are using articulated cars with two, three, or more carbody sections. Two-section articulated LRVs, which were the most common design when the first edition of the Track Design Handbook for Light Rail Transit was published, are now being purchased only for those LRT lines that require a 100% high-floor car to match high- platform stations. The development of LRVs with multiple-carbody sections (up to seven sections in the case of trams purchased in Budapest, Hungary, in 2007) was driven by the same issues as a century agoâcarrying more passengers with fewer operating employees. Multiple-carbody vehicles also have fewer motorized trucks per passenger and thereby provide substantial energy savings. Several North American systems are following this trend. Toronto ordered new five-section streetcars in 2008. Dallas Area Rapid Transit, following a trend started in Europe, modified older two-section, high-floor light rail vehicles to add a low-floor center section. New Jersey Transit has investigated adding two additional sections to their current fleet of three-section, 70% low-floor cars. Where two body sections meet, a turntable and bellows arrangement connects the sections, allowing continuous through passage for passengers from one end of the car to the other. In the case of high-floor LRVs, a single such arrangement, centered over a truck of conventional design, is used to connect two carbody sections. Low-floor LRVs require two such articulationsâone on each side of the center truck and center section of the carbodyâsince there is no room for the turntable above the special trucks required under low-floor cars. This usually results in a very short carbody section at each low-floor truck. Particularly in the case of low-floor LRVs, there are many variations on articulation joints, as each LRV manufacturer has devised its own specific design. These hardware variations can affect vehicle clearances since the pivot points of the articulation can be a considerable distance off of the centerline of sharply curved track. Variations in center section design also affect steering and relative roll, which might have some affect on vehicle curving and rail wear, thus influencing rail steel selection, track gauge, and track superelevation. The track designer has little control over this, but the problem is more difficult with low-floor vehicles using independently rotating wheels than with conventional high-floor vehicles equipped with solid axles. Existing systems contemplating a change to longer vehicles must consider overall train length and the impact that the revision might have on existing station platforms. Longer cars might require either a reduction of the number of vehicles in a train or lengthening existing platforms. One major LRT system in the United States initially designed their underground LRT stations for four-car trains of conventional two-section high-floor LRVs. When they added low-floor vehicles, trains had to be limited to three of the longer low-floor cars because the subway station platforms
Light Rail Transit Vehicles 2-7 could not be economically lengthened. Longer vehicles can affect other infrastructure and systems as well, particularly the layout of equipment within the light rail vehicle maintenance shop. There is a common misconception that articulated light rail vehicles can negotiate sharper curves than a rigid body car. This is not true. Rigid cars can negotiate curves that are as sharp, and even sharper, than an articulated vehicle. However, rigid cars are limited in both length and passenger capacity, primarily because the lateral clearances required in curves increase dramatically as the distance between the trucks increases. Where lateral clearances are not an issue, rigid body cars can be appreciably cheaper to procure and maintain than articulated cars of similar passenger capacity; however, this is a distinct exception to the normal circumstances. In North America, modern non-articulated light rail vehicles are used only in Philadelphia, Buffalo, and Toronto, but, as of 2010, those fleets, which are all high-floor designs, are in their third decade of operation. Outside of North America, the light rail system in Hong Kong and several cities in the former Soviet Bloc have continued to purchase rigid body cars, most likely for reasons peculiar to those systems. Therefore, while thousands of single unit, single-end trams, many of them of designs derived from the North American PCC car, still operate around the world, it is virtually certain that the LRVs for any new system will always be high-capacity, multiple-section, articulated cars. 18.104.22.168 High-Floor/Low-Floor LRVs 22.214.171.124.1 Introduction Getting passengers safely and expeditiously onto and off of light rail vehicles at stations has always been an issue. Time spent at stationsââdwell timeââcan be a significant percentage of the overall running time from terminal to terminal. For a conventional âhigh-floorâ light rail vehicle, with steps at the doors that are internal to the vehicle, the delays inherent in climbing up and down steps adds significantly to the dwell time. The various measures necessary to comply with the Americans with Disabilities Act Accessibility Guidelines (ADAAG) means even more delay before such LRVs can resume forward motion. Level boarding from the platform to the vehicle is clearly the best way to accommodate the mobility-challenged transit rider. Level boarding also reduces station dwell times by making it easier and quicker for all riders, mobility-challenged or not, to board and alight from the LRV. Because of these advantages, heavy rail metro systems have always used level or near level boarding from high level platforms. Following that example, several light rail systems built during the 1980s, in both North America and Europe, incorporated level boarding from high level platforms, largely eliminating the need for steps. The problem with high level platforms is that they usually can fit alongside of the tracks only if the light rail line is in an exclusive guideway such as a subway tunnel, an aerial structure, or a private right-of-way. High platforms that are the full length of the train (usually no less than 200 feet/60 meters for a two-car train) are generally impractical where the LRT guideway is in an urban street. Urban locations often also have insufficient space for vertical circulation elements to get passengers from street and sidewalk level up to a station platform that would usually be 3 feet
Track Design Handbook for Light Rail Transit, Second Edition 2-8 [0.9 meter] higher. Moreover, a two- or three-car long high platform will often be very intrusive on the urban streetscape, as well as quite expensive. Because of such issues, light rail systems that were constructed in the 1980s and early 1990s and included extensive operations in city streets typically used high-floor LRVs that were equipped with steps for patrons to board from sidewalk level. A variety of methods were used to get mobility-challenged persons on and off the vehicles, with mini-high platforms being the usual choice. However, these arrangements were generally less than fully satisfactory. Some means of providing level boarding for all riders without resorting to full-length high level platforms was desired. 126.96.36.199.2 Low-Floor CarsâGeneral In response to these issues, low-floor light rail vehicles were developed. In a low-floor car, either the middle portion or all the vehicle floor is positioned a short distance above top of rail. A typical dimension is 300 to 350 mm [about 11.7 Â¾ to 13 Â¾ inches]. This enables station platforms to be little more than sidewalks that are just slightly higher than normal above the street surface, making them much more practical for construction in congested urban areas. Since about 1995, the partial low-floor car (often called a â70% low-floorâ LRV) has become the preferred design for North American light rail transit systems that need level boarding from low platforms. The partial low-floor car has some middle portion of the LRV at the lower elevation while the ends of the car are at normal high-floor car elevation. The doors are usually all in the low-floor section of the car and the high-floor areas at the ends of the car are accessed by interior steps. The low-floor area usually represents approximately 70% of the total length of the car, hence the common name. (Bostonâs Type 8 LRVs are a notable exception; clearance limitations in the Green Line tunnels substantially restricted the truck center distance so that the low-floor portion of each car is only about 60% of the overall length.) One advantage of a 100% low-floor LRV is that the low profile of the cab and windshield increases the probability of eye contact between the operator and persons on the trackside. A corresponding advantage to a high-floor or 70% low-floor LRV is that the operatorâs higher seating provides a better view of the trackway ahead, which could be an advantage in some traffic situations. One possible issue with low-floor cars is that they maintain very close clearance to rails. With worn-out wheels, the vertical clearance between the underside of truck-mounted equipment and the plane of the top of rail can be a little as 35 mm [1 3/8 inches]. This could affect the use of some trackwork and signal system appliances mounted between the rails. The vehicle clearance also must be considered in design of tracks for hilly terrain, where the radius of the vertical curve over the crown of the street must be large. On one project, the low underclearance of the vehicle limited the height of discontinuous floating slabs that could be used, where maximum mass is needed for vibration control. 188.8.131.52.3 Low-Floor Car Truck Design The ends of the 70% low-floor car, including the operatorâs cabins, are generally at the same height as a high-floor car, allowing trucks of conventional design under the ends of the car. But it is not possible to use conventional trucks beneath the low-floor portions of the car because the
Light Rail Transit Vehicles 2-9 floor would be lower than the elevation of solid axles. The usual resolution is to use trucks that do not have conventional solid axles extending from wheel to wheel. Instead, the four wheels are each connected directly to a u-shaped frame that passes beneath the floor. Each wheel, lacking a mechanical connection to another, therefore rotates independently and is naturally called an independently rotating wheel (IRW). As an alternative to IRW trucks, at least one manufacturer has developed a truck using conventional solid axles connecting very small diameter wheels. This design also ramps the floor of the articulation body section slightly above that of the floor by the doors. However, small diameter wheels will have a smaller contact patch with the top of rail and thereby increase wheel/rail contact stresses, possibly increasing rail wear and corrugation rates. Because of constrained space, these special truck designs beneath the center sections of 70% low-floor LRVs are generally non-powered. Propulsion is provided only at the conventional trucks under the ends of the car. However, 100% low-floor cars must provide propulsion at trucks under the low-floor, and carbuilders have come up with several ingenious, albeit complex, methods for doing this. Because of this complexity, 70% low-floor cars using conventional power trucks have generally been considered more reliable than 100% low-floor cars. Nevertheless, the 100% low- floor LRV has been almost exclusively adopted for new vehicle purchases by in-street tramway type operations in Europe and also by some of the stadtbahn-type operations. As of 2010, the first 100% low-floor LRV specified in North America was being produced for Toronto Transit Commission. The Toronto cars are also specified to negotiate a 36-foot [11-meter] radius curve. The degree to which the carbuilder succeeds in meeting the Toronto requirements may radically change preferences for light rail vehicle design. As of 2010, the lowest 100% low-floor LRV was the Vienna Ultra-Low-Floor (ULF) car, with the floor a mere 200 mm [about 8 inches] above the top of the rail. The traction motors of the ULF car are mounted vertically within the articulation sections. As of 2010, this design has not been adopted elsewhere. The conventional trucks that are under the end body segments of 70% low-floor cars rotate with respect to the carbody. By contrast, the trucks under 100% low-floor LRVs generally do not rotate and are, for all practical considerations, rigidly fixed to the carbody. This configuration has resulted in vehicle designs that are radical departures from high-floor and partial low-floor designs and vehicles that have significantly different steering and curve negotiation characteristics. 184.108.40.206 Carbody Strength, Crashworthiness, and Mass 220.127.116.11.1 Introduction Up until about 1970, there were no codes or standards for the overall strength requirements of a transit vehicle carbody that were fully based in engineering principles. Beginning about that time, the usual requirement in specifications became that the carbody needed to accept, without structural failure, a longitudinal static âbuff loadâ equal to two times its own mass. This was known as the â2-g standard,â although it was never actually codified as a mandatory requirement except in the State of California. Under the 2-g standard, if the vehicle weighed 125,000 pounds [556 kilonewtons] it needed to have a minimum buff strength of 250,000 pounds [1,112 kilonewtons]. Naturally, the addition of more steel to make the carbody stronger also increased
Track Design Handbook for Light Rail Transit, Second Edition 2-10 its mass, with the result that new transit cars were much heavier than their predecessors. This extra weight had impacts on power consumption, structure design, and track design. 18.104.22.168.2 Crash Energy Management In response to those issues and following the lead of European LRV manufacturers, crash energy management (CEM) principles began to be incorporated into the design of light rail vehicles for North American use. CEM, which has been used in the automotive industry for decades, recognizes that designing the vehicle body to collapse in a controlled and predictable manner during a collision is better at minimizing injuries to the vehicle occupants than just merely making the carbody stronger. Beginning with a procurement of light rail vehicles for New Jersey Transit in the mid-1990s, CEM design principles began to replace the old 2-g criterion., ,  Subsequently, new standards were developed on both sides of the Atlantic. In Europe, European Norm (EN) 15227âRailway applicationsâCrashworthiness requirements for railway vehicle bodies, was implemented in 2008. A companion standard is EN 12663âRailway applicationsâStructural requirements of railway vehicle bodies. In North America, the American Society of Mechanical Engineers developed ASME RT-1â Safety Standard for Structural Requirements for Light Rail Vehicles. ASME RT-1, which is somewhat more restrictive and conservative than EN 15227, became effective in 2010. An updated edition is expected to be issued by ASME in 2014. As of 2010, for North American applications, either the ASME RT-1 or EN 15227 are voluntary (as was the old 2-g criterion) unless they are adopted and codified by either federal or state regulation. The European Norm and ASME RT-1 differ in several respects, and the latter is generally more rigid. For example, ASME RT-1 includes a collision scenario at 25 mph [40 km/h] while the equivalent EN 15227 test is performed at 25 km/h [16 mph]. Hence, vehicles designed to just meet the European Norms will likely not comply with ASME RT-1. The 100% low-floor cars for Toronto Transit Commissionâs legacy streetcar system were specified to meet EN 15227, with a slightly higher Category 4 speed, since ASME RT-1 existed only in draft form at the time of the procurement in 2008. The cars for Torontoâs Transit City program (underway as of 2010) were similarly specified under EN 15227 rather than changing from one voluntary standard to another. Since nearly all North American LRVs are designed and at least partially built overseas, the lack of consistency between European and North American standards increases procurement costs. The resultant heavier vehicles also have long-term ramifications concerning operating energy costs and loading and wear and tear on the track structure. As of this writing, it is unclear whether consistency between the North American and European standards will be possible. What does seem clear is that many of the lightweight LRVs that are common in other parts of the world are unlikely to be used in the United States, particularly on any project that utilizes federal funding. However, this situation is evolving. As of early 2011, revisions to ASME RT-1 that would eliminate all structural requirements that are inconsistent with European standards and may
Light Rail Transit Vehicles 2-11 unnecessarily increase the procurement costs are under consideration. Whether those changes will be adopted in whole or part cannot be predicted, and rail transit design practitioners must therefore keep current with evolving best practices. 22.214.171.124.3 LRV Bumpers A key feature of many modern LRVs is a front end bumper that is designed around crash energy management principles. The bumper typically extends from a few inches above the rails to the floor level of the LRV. The bumper is designed to rotate upward, revealing the LRV coupler. The coupler itself, which traditionally extended out an appreciable distance beyond the front of the LRV, is now hinged and can be folded back behind the closed bumper. The bumper conceals the traditional anticlimber as well as the coupler, but is not primarily intended to be merely cosmetic. Because of the CEM design, in the event of a collision, the bumper actually minimizes damage to any motor vehicles. It also makes it far less likely that an automobile would become wedged beneath the front of an LRV. Similarly, the bumper makes it more likely that a struck pedestrian will be pushed aside instead of being pulled beneath the front of the LRV. As of 2011, bumpers are not universal on new light rail vehicles, but it seems likely that they will become a common feature for any LRVs that have extensive operations in public streets. 126.96.36.199.4 Vehicle Mass As an example of what CEM principles can mean to carbody mass, it is useful to compare the 70% low-floor LRVs built for New Jersey Transit with those delivered to Santa Clara County (San Jose), California. The latter were constructed to the 2-g criterion under California PUC regulation 143-B while the former were designed around CEM principles. The same carbuilder produced both cars, and they have the same overall dimensions, performance, and capacity. The California car has a maximum wheel load at AW2 loading that is 540 pounds [245 kg] greater than that of the New Jersey LRV, a difference of 3.2 tons [2.9 metric tonnes] per car. The difference will result in appreciable propulsion energy cost savings over the life cycle of the New Jersey Transit car as well as less loading and wear and tear on the track. Table 2.2.1 compares the vehicle mass per unit of floor area between comparable 100% low-floor and 70% low-floor cars from selected European and North American cities. The difference averages about 100 kg/m2 [about 20.5 lb/ft2]. For an LRV that is 27.5 meters [90 feet] long and 2700 mm [8.9 feet] wide, this amounts to 7425 kg (16,390 lb) of additional weight that the vehicle must carry around through its entire service life, with implications for both energy consumption and loads applied to the track structure. In addition, the 100% low-floor vehicle may produce lower wheel/rail contact stresses than those produced by the 70% low-floor vehicle. One part of the difference in vehicle mass between low-floor and conventional articulated vehicles with solid axles is due to the deletion of the traditional truck. However, a major part of the difference is the different standards under which the cars were specified. Of the two North American 70% low-floor cars in Table 2.1, only the New Jersey Transit car was designed around CEM principles. Several of the European vehicles predate EN 15227 and EN 12663 and their degree of compliance with those standards is unclear. It is also very likely that most of these vehicles may not comply with ASME RT-1; therefore, for purposes of potential North American application, they may be irrelevant. Some might also argue that some of these vehicles are âtramsâ as opposed to âlight rail vehicles.â As noted in Chapter 1, European light rail operations typically donât make such distinctions between vehicle types.
Track Design Handbook for Light Rail Transit, Second Edition 2-12 Table 2.2.2 shows some of the characteristics of modern light rail vehicles operating in North American cities as of 2010. The table is not intended to be a comprehensive reference of every vehicle or every system now operating but rather an illustration of the rather wide array of vehicles that a track designer might encounter on any given project. Because light rail systems are constantly purchasing new cars and retiring older cars (and, in some cases, selling retired cars to other systems), the table is merely a snapshot of a dynamic condition. Track engineers working on designs for any transit system, including those listed below, should obtain up-to-date information on the agencyâs current LRV fleets before commencing any design. Table 2.2.1 Relative mass of 100% vs. 70% low-floor LRVs 100% Low-floor LRVs 70% Low-floor LRVs City Weight â lbs/ft2 [Mass â kg/m2] City Weight â lbs/ft2 [Mass â kg/m2] Lille 98  Kassel 93  Socimi 64  Valencia 107  Strasbourg 90  NJ Transit 114  Munich (Munchen) 99  Rostock 95  Chemnitz 71  Vienna (Wien) âTâ 100  Frankfurt 106  Portland 132  Turin (Torino) 96  Grenoble 133  Vienna (Wien) ULF 80  Bochum 100  Leipzig 107  Heidelberg 97  AVERAGE 88  AVERAGE 108 
Light Rail Transit Vehicles 2-13 Table 2.2.2 Light rail vehicle characteristics matrix (2010 data) CITY Carbuilder/Model DELIVERY YEAR WEIGHT AW0 lbs MAXIMUM WHEEL LOAD lbs LENGTH Feet CARBODY CONFIGURATION FLOOR LEVEL 1 Baltimore ABB 1989/1995 108,000 12,000 95 6-axle 2-carbody High 2 Boston KS Breda 1982 2000 85,000 86,300 9,350 9,500 74 74 6-axle 2-carbody High 50% Low 3 Buffalo Tokyu 1985 71,000 11,000 66â-10â 4-axle 1-carbody High 4 Calgary Siemens SD 160 1999/2008 89,600 9,800 81â5â 6-axle 2 carbody High 5 Charlotte Siemens S 70 2004/2008 96,800 10,700 93â6â 6 axle 3 carbody 70% low 6 Cleveland Breda 1982 91,300 9,800 80â 6-axle 2-carbody High 7 Dallas KS 1 KS 2 1998 2007 108,000 140,000 11,600 15,176 92â6â 123â6â 6-axle 2-carbody 8-axle 3-carbody High Low 8 Denver Siemens SD 100 Siemens SD 160 1995 2008 88,000 9,650 81â6â 6-axle 2-carbody 6-axle 2-carbody High 9 Edmonton Duewag U 2 Siemens SD-160 1982 2009 67,300 91,700 7,900 9,960 79â8â 81â4â 6 axle 2-carbody 6-axle 2-carbody High High 10 Houston Siemens S 70 2004 98,500 10,950 96â6â 6-axle 3-carbody 70% low 11 Los Angeles Nippon Siemens SD100 Siemens P2000 Breda 2550 1992 1993 1999 2008 98,000 98,000 89,000 10,700 10,700 9,970 89â 89â 90â 6-axle 2-carbody 6-axle 2-carbody 6-axle 2-carbody 6-axle 2-carbody High High High High 12 Minneapolis BBD Flexity 2004 99,180 10,940 94â 6 axle 3-carbody 70% low 13 New Jersey Kinki Sharyo BBD (DMU) 2000 2005 93,500 119,000 10,350 18,000 90â 102â 6 axle 3-carbody 6 axle 3-carbody 70% low 70% low 14 Norfolk Siemens S 70 2008 96,800 10,720 93â6â 6-axle 3-carbody 70% low 15 Philadelphia City Suburban 1982 1982 57,300 59,500 6,200 50â 4-axle 1-carbody Single end Double end High High 16 Phoenix Kinki Sharyo 2008 102,000 11,100 91â5â 6-axle 3-carbody 70% low 17 Pittsburgh Duewag /CAF CAF 1984/R2005 2004 97,000 100,000 10,500 10,740 84â8â 84â8â 6-axle 2-carbody 6-axle 2-carbody High High 18 Portland Bombardier Siemens SD 660 Siemens S 70 Skoda Inekon 1986 2000 2009 2006 92,150 109,000 99,000 56,000 10,200 11,700 10,990 9,813 89â1â 92â0â 96â6â 66â0â 6-axle 2-carbody 6-axle 2-carbody 6-axle 3-carbody 4-axle 3-carbody High High 70 % low 50% low 19 Sacramento Siemens SD 100 CAF UTDC 1991 2003 1989 77,175 93,735 98,700 8,690 10,190 10,740 79â6â 83â9â 88â6â 6-axle 2-carbody 6-axle 2-carbody 6-axle 2-carbody High High High 20 St. Louis Siemens SD100-1 Siemens SD100-2 1993 2001 90,390 93,000 10,080 10,290 89â5â 89â5â 6-axle 2-carbody 6-axle 2-carbody High High 21 Salt Lake Siemens SD 100 UTDC Siemens S70 2002 1989 2010 88,000 98,700 TDB 9,650 10,740 TDB 81â5â 88â6â TDB 6-axle 2-carbody 6-axle 2-carbody 6-axle 3-carbody High High 60% Low
Track Design Handbook for Light Rail Transit, Second Edition 41-2 Table 2.2.2 Light rail vehicle characteristics matrix (2010 data) (continued) CITY Carbuilder/Model DELIVERY YEAR WEIGHT AW0 lbs MAXIMUM WHEEL LOAD lbs LENGTH Feet CARBODY CONFIGURATION FLOOR LEVEL 22 Seattle Kinki Sharyo 2008 102,000 11,200 95â0â 6-axle 3-carbody 70% low 23 San Diego Siemens U2 Siemens SD100 Siemens S 70 1989 1996 2005 71,800 88,000 95,500 8,250 9,650 10,540 79â8â 81â5â 90â7â 6-axle 2-carbody 6-axle 2-carbody 6-axle 3-carbody High High 70% low 24 San Francisco Breda 1998 78,000 8,630 75â0â 6-axle 2-carbody High 25 San Jose Kinki Sharyo 2001 99,980 10,890 90â0â 6-axle 3-carbody 70% low 26 Toronto UTDC CLRV UTDC ALRV 1982 1987 51,000 78,600 8,612 8,750 52â6â 77â6â 4-axle 1-carbody 6-axle 2-carbody High High No attempt was made to include vintage or heritage streetcars in Table 2.2.2 since they come in so many versions. Further, since even the newest of the vintage PCC streetcars still operating in the United States will be 60 years old in 2012, it is an open question how long the use of any such vintage equipment in daily revenue service can be sustained. Modern low-floor streetcars, which can directly comply with ADAAG without resorting to wheelchair lifts and/or ramps and which could also easily be constructed with a faux antique appearance, would seem to be a more rational choice for new streetcar programs. As is the case with any modern light rail car, the track designer should inquire as to the characteristics of any vintage streetcars that might be proposed to occasionally operate over the system so they can be accommodated in the design of both track alignment and trackwork. 2.3 VEHICLE CLEARANCES This article discusses the dimensional characteristics of the light rail vehicle. This includes not only the static vehicle at rest, but also the additional dynamic movements the LRV can make due to both resiliency and possible failures in the vehicle suspension system. The result is a definition of the vehicle dynamic envelope (VDE). The VDE, plus additional factors, defines the track clearance envelope (TCE), which sets the minimum distances between the centerline of track and any infrastructure alongside of the track as well as the minimum distances between tracks. Because the TCE includes elements that are unrelated to the vehicle, it will be discussed in detail in Chapter 3. 2.3.1 Vehicle Clearance Envelopes Clearance standards for various types of railroad cars are well documented by the use of graphics or âplates.â For railroad equipment, one standard is the common âPlate C.â Any car whose static dimensions fit within the limits established on Plate C can travel virtually anywhere on the North American railroad system. Transit systems do not have similar national standards. Therefore, transit vehicle manufacturers must develop vehicles that fit within the clearance requirements of the system for which the car is intended. Conversely, transit system designers should, whenever possible, configure the infrastructure so as to allow clear passage of as broad a universe of candidate LRVs as possible. While manufacturers can, in theory,
Light Rail Transit Vehicles 2-15 build cars to any dimension, it is usually more economical to choose vehicles that are already in production or have at least been engineered. Therefore, the facility designer of a new system should establish a composite vehicle clearance envelope that accommodates vehicles from several manufacturers to maximize competitive bidding and then design the system to accommodate those clearances. The composite vehicle clearance envelope considers both the static and dynamic outlines of the vehicles under consideration. The static outline is the cross-sectional shape of the car at rest on tangent level track. The dynamic outline includes the allowable movement in the suspension system due to vehicle pitch, roll, yaw, and curving characteristic. The manufacturer develops the actual dynamic outline for their transit vehicle so as to fit within the ownerâs clearance restrictions. In addition, as the vehicle passes through curved track, the lateral excursions of the carbody will vary depending on the static plan shape of the vehicle, the distance between the trucks, and the amount of curvature. To establish clearances along the right-of-way, a vehicle dynamic clearance envelope must also be developed. Using the vehicle dynamic outline along with the associated track components, track tolerances, wear limits of the components, and a running clearance zone, the track clearance envelope can be established. LRV procurement specifications may include the following requirements related to clearances: â¢ A dynamic envelope as established in the projectâs Manual of Design criteria. â¢ Minimum clearance under any car component under worst wheel and suspension condition. â¢ The minimum track curve radius. â¢ The maximum allowed curve offset and minimum carbody shift in the tightest track curve radius under worst track conditions and/or with maximum superelevation. â¢ Demonstration that the horizontal clearance (gap) and vertical match to station platforms is in compliance with ADAAG. The latter require that passengers step down from the car floor onto the station platform when alighting from the vehicle even with the worst situation of wear on both wheels and rail. â¢ Gap between vehicle door sill and platform edge, which may affect wheelchair access. Trackform design may influence the clearance envelope; ballasted track may shift with time while direct fixation and embedded track will not. For additional information on vehicle clearances, particularly the track and wayside issues that affect the structure gauge and the swept path of the LRV through curves, refer to Chapter 3, Article 3.4. 2.3.2 Vehicle Static Outline The static outline of an LRV is based on plan and cross-sectional views showing its dimensions at rest, including many elements as discussed below.
Track Design Handbook for Light Rail Transit, Second Edition 2-16 188.8.131.52 Vehicle Length When considering the length of a light rail vehicle, it is important to distinguish between the actual length of the carbody and its length over the coupler faces as follows: â¢ Over Coupler FaceâThe coupler is the connection between LRVs that operate together. It extends beyond the front of the car structure. The length over the couplers becomes a consideration for determining the requisite length of facilities such as station platforms and storage tracks for coupled and uncoupled trains. â¢ Over Anticlimber or BumperâThe anticlimber is a ribbed bumper at floor elevation positioned at the structural end of the car. In the event of a collision between two LRVs, the anticlimbers on each car will interlock and, as the name implies, thereby reduce the possibility of one LRV climbing over the floor level of the other during a collision. The length of the vehicle over the anticlimbers was traditionally used to determine clearances, but the current generation of light rail vehicles often conceals the anticlimber behind a movable bumper. Regardless of whether the LRV is equipped with a bumper or a visible anticlimber, the positions of the outer corners of the device with respect to the track centerline and the vehicle trucks will often define the swept path of the vehicle toward the outside of any curve. When considering the length of a light rail vehicle, it is important to distinguish between the actual length of the carbody and its length over the coupler faces. Another important longitudinal dimension, one that generally does not affect clearances but can be a significant design factor, is the distance from the leading edge of the first door on the LRV to the rear edge of the last door on the car (or the last door on a multiple-car train). Occasionally, while doing track alignment at a station, providing a segment of tangent track that is the full length of a train may not be possible. However, if only the door-to-door dimension is used to define the ADAAG-compliant platform edge, it may make the difference between being able to provide a station at a key location versus having no station at all. This topic is discussed further in Chapter 3. 184.108.40.206 Distance between Truck Centers The distance between adjacent truck pivot points determines the overhang of a carâs midsection for given track curvature. This âtruck centerâ distance is a key factor in determining the extent of the vehicleâs swept path toward the inside of the curve. A vehicle with a long truck center distance will have a greater âmid-ordinateâ clearance excursion than one with a shorter truck center distance. Conversely, a vehicle that has a truck center distance that is relatively short will usually have a large âend-overhangâ clearance to the outside of the curve. In the case of the center truck of a low-floor LRV, the pivot points are not coincident with the center of the truck. As a result, they will be located some distance to the outside of the centerline of the track as the car passes through a curve, affecting both the mid-ordinate and end-overhang distances. (Notably, during curving, longitudinal and transverse forces may induce rotation of the center truck/carbody section, increasing angle of attack, gauge face wear, and noise and may affect ballasted track alignment stability.) 220.127.116.11 Distance between End Truck and Anticlimber or Bumper This dimension and the carbody end taper (if any) determine the overhang of the front of the car toward the outside of the curve for a given track curvature.
Light Rail Transit Vehicles 2-17 18.104.22.168 Carbody Width The width of the LRV carbody is determined by several factors: â¢ In the case of any LRV that will be operating in mixed traffic in a street, it generally should comply with the legal maximum widths for motor vehicles. There can be some latitude on this since, unlike a large rubber-tired vehicle such as a truck, the path of the LRV is absolutely predictable. See Chapter 12 for additional discussion on this point. â¢ The transit agency requirements regarding the total number of passengers seated versus standing, the number and arrangement of seats, specified human factors for the width of the single seats and double-seats, and allowances for wheelchairs of standard size. â¢ Total vehicle wall thickness. â¢ In the case of an existing LRT system procuring new vehicles, any existing clearance restrictions may limit several vehicle dimensions, including width. Vehicle procurement specifications for existing systems replacing legacy rolling stock typically need vehicles no wider than 8.33 to 8.83 feet [2540 to 2690 mm] in width so as to match existing clearances. In some cases, the sides of the carbody are tapered, rather than vertical, so that the car is narrower at the ceiling than it is at floor level. This taper partially compensates for vehicle roll and keeps the dynamic clearance envelope smaller. The widest point on some rail cars is actually located at window sill level so as to maximize shoulder room for seated passengers. A few North American systems can accommodate wider than normal light rail vehicles. The Breda LRVs in San Francisco are 9.0 feet [2745 mm] wide. Clevelandâs Breda LRVs are 9.3 feet [2835 mm] wide while Baltimoreâs ABB light rail vehicles, which were designed to operate on tracks shared with freight trains, are 9.6 feet [2925 mm] wide. Trams as narrow as 2400 mm [7.9 feet] are operated on some European systems where close clearances cannot allow wider cars. Such narrow cars are not recommended for new operations since their passenger capacity is significantly less than standard width vehicles. 22.214.171.124 Carbody End Taper The plan view configuration of the end of a light rail vehicle is usually not square. Instead, it is tapered, usually over the length of the operatorâs cabin. The principal reason for this is neither aesthetics nor aerodynamics but rather to reduce the dynamic excursions of the ends of the LRV as it passes through curved track. Figure 2.3.1 illustrates a typical three-section articulated LRV passing through a tight radius curve. Note how the amount of taper at the ends of the car reduces the clearance requirements to the outside of the curve. If the carbody maintained the same width all the way to the end of the car, the vehicle excursions to the outside of the curve would be much greater. Some vehicles have even more taper so that clearances to the outside of the curve are actually controlled by the carbody width at the rear of the operatorâs cab and not at the nose of the car. The reduced width of the front of the cab still provides sufficient room for the operatorâs dashboard and other equipment.
Track Design Handbook for Light Rail Transit, Second Edition 2-18 Figure 2.3.1 Three-section 70% low-floor LRV in an 82-foot [25-meter] radius curve The ideal situation clearance in curves on a double-track route is to design the end taper and select the truck centers and pivot point locations so as to make the mid-ordinate and end- overhang clearances at equal distances from the track centerline. This permits placement of the catenary poles exactly halfway between the two tracks. The designers of the vehicles for one rail transit project were able to balance these so that on an 82 foot [25 meter] radius curve, the end- overhang and the mid-ordinate differed by only about Â¼ inch [6 mm]. This new LRV also fits within the clearances of the PCC streetcars that formerly operated on a portion of that reconstructed and expanded light rail system. 126.96.36.199 Other Static Clearance Factors On most light rail vehicles, the overall width is governed by the external rear view mirrors, which are mounted on the corners of the car outside of the motormanâs cabin. Notably, the mirrors are only a clearance control at the elevation where they are mounted. Trackside objects that are higher or lower than the mirrors can sometimes be placed closer to the track. Some LRVs are now equipped with rear facing cameras, which permit the operator to monitor multiple locations along the length of the vehicle or train from a display screen on the dash. There usually will be several cameras on each side of the car with some facing forward as well as backwards. Some jurisdictions prohibit video displays that can be seen by a motor vehicle operator, and waivers of those regulations may be required. The cameras are much smaller than the mirrors they replace and each might extend out beyond the face of the vehicle only half the distance required for a mirror, thereby making the clearance outline of the vehicle appreciably narrower. The cameras are also mounted somewhat to the rear of the motormanâs cabin and so do not widen the vehicle body at the ends of the car. This can significantly reduce the âend-overhangâ vehicle clearance requirements to the outside of curved track, making it possible to take full advantage of the LRV body end taper. The doors on some light rail vehicles have thresholds which project out some distance beyond the sides of the carbody. These are sometimes designed to be âsacrificialâ should they collide with a platform edge. Projecting thresholds are sometimes seen on systems that have a mixed vehicle fleet where the actual width of one or more series of rail cars are narrower than others. This permits both wide and narrow vehicles to service the same platforms.
Light Rail Transit Vehicles 2-19 The geometric center of the plan view of a rail vehicle truck in curved track will not be coincident with the centerline of the track, but rather shifted some distance toward the inside of the curve. The magnitude of this shift will vary depending on the axle spacing of the truck, the radius of the curve, the lateral position of the truck relative to the rails, and any skew the truck may have assumed relative to the track. For LRV trucks with axle spacings less than about 6 feet 6 inches [2.0 meters] the shift is negligible for curves with radii greater than 300 feet [91 meters]. It can be a factor for sharper curves and/or longer axle spacings. 2.3.3 Vehicle Dynamic Envelope/Outline The dynamic outline of the car is more significant to the track alignment designer than the static outline. The vehicle dynamic envelope (VDE) of an LRV describes the maximum space that the vehicle may occupy as it moves along the track. The dynamic outline or âclearance envelopeâ includes many factors due to the normal actions of the vehicleâs suspension system, such as carbody roll (side sway) and lateral movement between stops. The dynamic outline also includes lateral freeplay between wheels and rail with both in their maximum wear condition as well as abnormal conditions that may result from failure of suspension elements (e.g., deflation of an air spring). The development of the VDE is typically the responsibility of the vehicle designer and begins with the cross-sectional outline of the static vehicle. The dynamic outline of the vehicle is then developed by making allowances for carbody movements that occur when the vehicle is operating on level tangent track. These movements represent the extremes of carbody displacement that can occur for any combination of rotational, lateral, and vertical carbody movements when the vehicle is operating on level tangent track. The following items are typically included in the development of the VDE: â¢ Static vehicle outline â¢ Dynamic motion (roll) of springs and suspension/bolsters of vehicle trucks â¢ Vehicle suspension side play and component wear â¢ Vehicle wheel flange and radial tread wear â¢ Maximum truck yaw (fishtailing) â¢ Maximum passenger loading â¢ Suspension system failure â¢ Wheel and track nominal gauge difference â¢ Wheel back-to-back mounting and maintenance tolerance In addition, some projects include allowances for the following: â¢ Rail fastener loosening and gauge widening during revenue service â¢ Dynamic rail rotation However, since these two factors are not under the control of the vehicle supplier and could also vary considerably with trackform, it is recommended that these factors not be included in the VDE
Track Design Handbook for Light Rail Transit, Second Edition 2-20 but instead be addressed by the track designer as part of the track construction and maintenance tolerances. If the vehicle designer does include track factors in the VDE, that fact needs to be clearly documented. Whoever adds the track tolerances must utilize relatively liberal maintenance tolerances and not the typically stringent construction tolerances in the determination of the VDE. Typical values for vehicle-based maintenance factors include the following: â¢ Nominal wheel gauge-to-track gauge freeplay: 0.405 inch [10.5 mm] â¢ Lateral wheel flange wear: 0.3 inch [7.5 mm] â¢ Vertical radial wheel wear: 1 inch [25 mm] The VDE is usually represented as a series of exterior coordinate points with the reference origin at the track centerline at the top-of-rail elevation. The static vehicle outline is generally not used in track design except for the establishment of station platforms and associated station trackwork design at these locations. The dynamic outline is compiled for tangent track with zero cross-slope in the rails. Track curvature, superelevation, and maintenance tolerances are considered separately and will be discussed in Chapter 3 at Article 3.3.4. Any project will actually have two dynamic envelopes to consider: â¢ The first will be a proposed or provisional dynamic envelope that is developed as a part of the LRV procurement specification. This will be based on the characteristics of the hypothetical composite LRV. The procurement specification will typically include language such as: âThe vehicle shall be designed to operate within the dynamic envelope under all condition of wear or failure other than structural failures.â â¢ The second envelope will be the actual dynamic envelope for the vehicle purchased. It will be provided by the selected vehicle manufacturer and indicate its conformance to the specification (or, in some cases, situations where a waiver of some portion of the provisional envelope is requested). In Figure 2.3.2, the outer lines indicate the dynamic envelope stipulated in one procurement contract while the inner dotted lines show the supplierâs compliance with the specified limits. The vehicle dynamic outline is merely a two-dimensional cross section of the car illustrating its extreme movement due to factors related to the car itself. As the vehicle and its dynamic envelope pass along the track, they generate a three dimensional shape known as the âswept path.â The characteristics of the swept path will be discussed in Chapter 3 at Article 188.8.131.52.1.
Light Rail Transit Vehicles 2-21 Figure 2.3.2 Typical LRV dynamic envelope
Track Design Handbook for Light Rail Transit, Second Edition 2-22 184.108.40.206 Vehicle Components Related to Vehicle Dynamic Envelope The vehicle dynamic envelope is influenced by both the as-fabricated characteristics of the vehicle, particularly its suspension system, and possible wear and/or failure of vehicle subassemblies. These factors include â¢ Primary/secondary suspension systems â¢ Maximum roll/lean/sway â¢ Maximum lean due to total failure of all truck components â¢ Wheel tread and flange wear Air springs (also known as air bags) are a common element in the secondary suspension system. They serve multiple functions, including keeping the floor both reasonably level and matched to the station platform height regardless of the number of passengers on board. The air springs on each truck are interconnected by lines which include balancing valves. The balancing valves detect changes in pressure in one air bag versus the other and automatically make adjustments. In the case of a sudden loss of pressure in one bag, the balancing valve will automatically deflate the other. This prevents a sudden change in the LRVâs center of gravity that might otherwise result from one side of the carbody abruptly rising to the mechanical limitsâan event that could unload one or more wheels and lead to a derailment or cant the vehicle excessively and conflict with tunnel wall appurtenances. 220.127.116.11 Track Components Related to Vehicle Dynamic Envelope Various issues related to the track will affect the magnitude of the dynamic excursions of the LRV. These include the following: â¢ Track superelevation/crosslevel â¢ Wheel gauge-to-track gauge lateral clearance/freeplay â¢ Construction tolerances and maintenance tolerances for track surface, crosslevel, and alignment â¢ Maintenance tolerances for rail head wear and gauge face wear Typically, the only factor in the list above that is included in the vehicle dynamic envelope would be the design value of freeplay between the track gauge and the wheel gauge. The other factors are not under the control of the vehicle supplier and therefore should instead be addressed by the track designer. Sometimes the vehicle supplier will include track-related factors in its calculated VDE, but those numbers can include unrealistically stringent assumptions as to the track maintenance tolerances that can be achieved. So as to avoid double-counting such issues, the track designer should back out any track-related tolerances that may be in the vehicle supplierâs VDE and substitute values that are consistent with the transit agencyâs maintenance track maintenance standards. 18.104.22.168 Vehicle Clearance to Wayside Obstructions and Other Tracks It is not unusual to have clearance restrictions on an LRT line that cannot be either simply or economically altered. In such cases, the track designer should coordinate with the vehicle and structural designers to ensure that the vehicle dynamic envelope considers these limitations so that adequate clearances result. Vehicle dynamics are governed by the carâs suspension system(s) and, therefore, indirectly by numerous factors of track and vehicle interaction. For multiple-track situations, multiple clearance envelopes must be considered. Overlapping of the
Light Rail Transit Vehicles 2-23 vehicle dynamic envelopes from adjacent tracks obviously must be avoided. The resulting requirements will dictate minimum track centers and running clearances for tangent and curved track, including construction and maintenance tolerances as input to the track alignment calculations. In general, the absolute minimum tangent track centers for vehicles of normal width (e.g., 2650 mm / 8.7 feet) for rigid trackforms (direct fixation or embedded) are 13 feet 6 inches [about 4.15 m] with a catenary pole between the tracks. If the poles are outboard of the tracks, 11 feet [about 3.35 m] is the typical minimum spacing. Tangent track center spacing for ballasted track is typically 6 inches [15 cm] greater than those for rigid trackform track due to greater allowances for construction tolerances and shifting of the tracks over time. Track curvature and superelevation increase these dimensions. These issues are discussed further in Chapter 3, Article 3.8. 22.214.171.124 Platform Clearances One clearance requirement that can be difficult for vehicle manufacturers is keeping the dynamic envelope at platform height from intersecting the edge of the platform. Since ADAAG requires the horizontal gap between the static vehicle and the platform to be 3.0 inches [76 mm] or less, the fully dynamic vehicle might actually strike the platform. In the case of high-floor LRVs adjacent to a high level platform, interference between the platform edge and the vehicle dynamic envelope is virtually inevitable. This is largely because the vehicle roll center is typically about 2 feet [approximately 0.6 meter] below the platform surface. However, LRVs virtually never actually strike a high platform edge because it is extremely unlikely that the vehicle and track factors that might lead to full excursions to the limits of the dynamic envelope will ever occur simultaneously. The use of a rigid trackform (e.g., either embedded or direct fixation track) and/or scrupulous maintenance of ballasted track surface and crosslevel and horizontal alignment can minimize the track contribution to vehicle dynamics. On the vehicle side, thresholds that project beyond the face of the vehicle and are designed to be âsacrificialâ can minimize damage to both the vehicle and the platform edge. Low-floor LRVs have very little chance of striking a low platform edge because the platform surface is typically a few inches [centimeters] below the carbody roll center as shown in Figure 2.3.2. Hence, while the platform clearance might still be reduced by carbody lateral translation, roll will not increase the encroachment. See Article 2.9 in this chapter and Chapter 3, Article 3.8.3 for additional discussion concerning the interface between LRVs and station platforms. 126.96.36.199 Pantograph Height Positions When discussing the height of a light rail vehicle, two conditions must be considered: â¢ RoofâThe roof of an LRV is typically curved, with the highest dimension at the car centerline. However, the LRV pantograph, when deployed, obviously establishes the maximum car height. In the case of high-floor LRVs, the pantograph is the highest point on the car even when in the âlock-downâ position. Low-floor LRVs, which have much more equipment on the roof (since there is little room under the floor), sometimes have some equipment sitting higher than the pantograph. However, the overall height of the
Track Design Handbook for Light Rail Transit, Second Edition 2-24 car with the pantograph locked down is typically only of concern in the design of maintenance shop infrastructure, such as the entrance door to a paint booth, where the LRV would usually be pushed or towed by other equipment. Lock-down clearances would only be a consideration along revenue service track if the LRV has âoff-wireâ operating capability. â¢ Pantograph OperationâLight rail facility designers are typically interested in the absolute minimum clearance between the top of the rail and an overhead obstruction, such as a highway bridge. This dimension must accommodate not only the pantograph when operating at some working height above lock-down, but also the depth of the overhead contact wire system. The minimum pantograph working height above lock-down includes an allowance for pantograph âbounceâ so that lock-down does not occur accidentally. Maximum pantograph height is typically the concern of only the vehicle and overhead catenary system (OCS) designers, unless the light rail guideway must also accommodate railroad freight traffic and attendant overhead clearances. If railroad equipment must be accommodated, the clearance envelope will be dictated by AREMA-recommended practices, state regulations, and the standards of the freight railroad involved. The minimum height of the trolley wire above a freight track will be much higher than the minimum height above an LRT-only track. See Articles 3.8.4 and 11.5.3 for additional discussion of this topic. 2.4 VEHICLE-TRACK GEOMETRY The most demanding light rail transit alignments are those running through established urban areas. Horizontal curves must be designed to suit existing conditions, which can result in curves below a 25-meter (82-foot) radius. Vertical curves are required to conform to the existing roadway pavement profiles, which may result in exceptionally sharp crest and sag conditions. LRVs are specifically designed to accommodate severe geometry by utilizing flexible trucks, couplings, and mid-vehicle articulation. Articulation joints, truck maximum pivot positions, coupler-to-truck alignments, vehicle lengths, wheel set (axle) spacing, truck spacing, and suspension elements all contribute to vehicle flexibility. The requirements for the truck to accommodate, within reasonable limits, free movement in three planes are defined in the vehicle procurement specification. Guidelines for these factors are included in the APTA Manual of Standards and Recommended Practices for Rail Passenger Equipment, RP-M-009-98 Recommended Practice for New Truck Design.  The torque the truck exerts against free turning is critical for resistance against derailment. Light rail carbody/truck connections that use either a ball bearing slewing ring or a king pin, without side pads, generally have good horizontal free movements. Air spring suspensions generally provide satisfactory free roll and yaw movements. Truck-related submittals from the vehicle supplier may include proof of compliance with the Truck Swivel Index (TSI), a factor calculated in accordance with Koffmanâs Formula, a guideline developed by British Rail in the 1970s. The track designer must take into account the vehicle characteristics defined in the articles below when developing alignments. The values associated with these characteristics are developed and furnished by the vehicle manufacturers. The manufacturer of vehicles supplied to existing systems must meet the existing minimum geometrical requirements of the system.
Light Rail Transit Vehicles 2-25 2.4.1 Horizontal CurvatureâMinimum Turning Radius of Vehicle The minimum turning radius is the smallest horizontal radius that the LRV can negotiate. In some cases, the value may be different for a single LRV versus two or more coupled into a train or for a fully loaded LRV versus an empty one. However, the inclusion of curves in a track layout that can only be negotiated by a single vehicle is absolutely not recommended since operating personnel may not remember the restriction, particularly during an emergency situation such as when an inoperable LRV must be pushed off the revenue line by its follower. The vehicle procurement specifications will therefore typically stipulate only the minimum radius that multiple-car trains of LRVs must be able to negotiate. The LRV supplier will typically be required to provide submittals that demonstrate that the proposed vehicle can negotiate the tightest curve under full design load without any binding in the trucks, articulation joints, or couplers. A specification for one LRV procurement stipulated: The coupler and draft gear shall allow under emergency conditions, a three vehicle train with an AW3 passenger load, operating at degraded dynamic performances, to push or tow an inoperable similar train consist loaded to AW3 without damage to the coupler, over all grades and curves of [the system]. Often, the minimum operable multi-vehicle train length requirement will be much longer than the consists actually required for revenue service. This is so as to accommodate shop and yard movements and other exigencies. Such long consists will occasionally have some impact on track alignment. One vehicle specification stipulated: The vehicle shall be capable of multiple unit operation in consists up to six vehicles. A normal operation is up to three vehicles. 2.4.2 Vertical CurvatureâMinimum Sag and Crest Curves The minimum vertical curvature is the smallest vertical curve radius that the LRV can negotiate. The maximum sag and crest values are typically different, with the sag value being more restrictive. Vehicle builders describe vertical curvature in terms of either the radius of curvature or as the maximum angle in degrees through which the articulation joint can bend. The trackway designer must relate those values to the parabolic vertical curves typically used in alignment design. When new vehicles are procured for an existing system, they must be able to negotiate the most restrictive current track condition. Conversely, when existing vehicles will be used on a new extension of an existing system, the new track must accommodate the existing vehicleâs capabilities. The vehicle procurement specification will include requirements related to specific track conditions, be they existing or proposed. 2.4.3 Combination Conditions of Horizontal and Vertical Curvature The car builder may or may not have a graph that displays this limitation. If a route design results in significant levels of both parameters occurring simultaneously, the design should be reviewed
Track Design Handbook for Light Rail Transit, Second Edition 2-26 with potential LRV suppliers to establish mutually agreeable limits. The following is a typical example from one vehicle specification: Reverse vertical curve: A two-vehicle consist shall be capable of negotiating a reverse vertical curve section involving: first, a crest of 250 m [820 feet] and a sag of 350 m [1150 feet], separated by a tangent section of 13 m [43 feet]; and second, a crest and sag curve of 500 m [1640 feet] separated by no tangent track. Compound curves: A two-vehicle consist shall be capable of negotiating a compound [horizontal and vertical] curve involving: first, a 25 m [82 feet] radius horizontal curve and a 500 m [1640 feet] radius vertical curve, either crest or sag; second, a 27 m [89 feet] radius horizontal curve and a 350 m [1150 feet] radius sag curve; and third, a 29 m [95 feet] radius horizontal curve and a 250 m [820 feet] radius crest curve. Alternatively, a set of plan and profile drawings can be included as an appendix in the vehicle procurement documents giving complete geometric information, including gradients, civil design speeds, and track superelevation. 2.4.4 Vertical AlignmentâMaximum Grades The maximum grade that a light rail vehicle can ascend is limited by the electrical and mechanical limits of the propulsion system. The maximum grade that an LRV can descend is limited by the braking system. Both climbing and descending are constrained by the limits of adhesion between the wheels and the rails. Tractive effort between wheels and rails is dependent on the amount of vehicle weight on powered axles and, generally speaking, light rail vehicles that have all axles powered can more reliably climb steep grades than cars with some number of non-powered axles. Braking is virtually always available on all wheels, powered or not. However, descending steep grades can sometimes be a greater issue than climbing the same hill since a high percentage of the braking effort is required to slow the vertical descent and hence not available to retard horizontal movement. Generally, grades of unlimited length up to about 6% to 7% are not a problem for any light rail vehicle. Above that the operational impacts should be reviewed, including: â¢ The tractive and braking characteristics of the LRV in normal operation. â¢ Situations where a disabled LRV (or train of LRVs) is being pushed or towed by another train. The critical situation might not be pushing the disabled vehicle train up the grade, but rather controlling the descent when going down the hill. â¢ The possibility of any lubricants on the rail running surface, particularly grease that might have migrated from some nearby curve and unintentionally lubricated the rail running surface. Grades of up to 10% are possible, and some legacy streetcar lines, using cars with all axles powered, were even steeper. However, wheel-to-rail slippage can occur on any gradient during inclement weather conditions, such as when snow, ice, or wet and/or oily leaves are on the rail.
Light Rail Transit Vehicles 2-27 Slippage may result in rail burns during both acceleration and braking and wheel flats during braking. Light rail vehicles have always been equipped with sanders, activated by the operator to drop dry sand on the rail and thereby increase friction between wheel and rail. Modern vehicles with slip/slide detection will also automatically dispense sand when required. Sand will therefore accumulate along steeply graded tracks and also in station areas. The sand will mix and bond with other contaminants on the trackway (including rail lubricants and friction modifiers) and wash downgrade to the lowest points on the track structure. Ideally, the track design should provide for this contamination to wash away harmlessly before it can become a path for stray currents and corrosion, but a comprehensive housekeeping program to keep accumulated sand from becoming a problem is generally necessary. Combinations of steep gradients, small radius horizontal curves, and sharp vertical curves are found on many light rail lines. One LRT line in the eastern United States has an 82-foot [25 meter] horizontal curve on a down grade of 6% followed by a sag vertical curve with a radius of about 1640 feet [500 meters]. At the other end of that vertical curve is a short up grade of 7% leading to a crest vertical curve followed by the junction turnout to another route. Legacy streetcar lines often had alignments that were even more convoluted. While such tortuous track alignments are possible, they tax the capabilities of the vehicle, slow down transit operation, require much higher than normal maintenance, are usually sources of high noise and vibration, and cause poor ride quality. They therefore are generally not recommended unless absolutely nothing better is possible within the project budget. The track alignment designer should work closely with all other project disciplines, including the vehicle engineers, so as to be certain that any complicated track alignments do not create any intractable problems for other members of the design team. 2.4.5 Maximum Allowable Track Twist Truck equalization refers to the changes in individual wheel loading that occur when one wheel on a two-axle truck moves above or below the plane of the other three wheels. If a wheel is unloaded significantly, it may climb the rail and derail. The truck needs to be sufficiently limber so as to maintain roughly equal vertical load on all four wheels regardless of any such twist and avoid unloading. Several situations can result in twist that can unload one wheel of a truck: â¢ Misalignments in the track surface such as a low rail joint that has dropped some measurable distance below the plane of the rails. â¢ Track superelevation transitions where the profile of one rail is rising relative to the other. â¢ Deliberate twist in tangent or curved track such as an embedded track section where normally crowned pavement (required for drainage) transitions to either a level or superelevated section. LRV truck equalization must be compatible with the maximum expected track vertical surface misalignment to prevent conditions that can cause a derailment. The following is a typical
Track Design Handbook for Light Rail Transit, Second Edition 2-28 specification for the maximum wheel unloading when one wheel is leaving the horizontal planeâ such as when being lifted by the outer rail on spiral curve with superelevation: Lifting or lowering any wheel on a truck 38 mm (1.5 inches) shall not cause the load to change on any wheel of that truck by more than 50% with the vehicle on level tangent track and under an AW0 load. Loss of contact shall not result between any of the wheels and the rail when raising or lowering one wheel on a truck up to 50 mm (2 inches). The dimensions above provide a considerable factor of safety so as to avoid routinely loading the truck to its mechanical limits and are unlikely to occur in track. For example, an LRV truck with axle centers of 6 feet [about 1.8 meters] that is negotiating a spiral with a superelevation raise rate of 0.20% (about Â¾ inch in 31 feet or 19 mm in 9.45 meters), will have the leading outside wheel raised by only 0.15 inches [4 mm]. Even if the track surface had substantially deteriorated, it is unlikely that track twist over the length of a truck would ever be more than Â¾ inch [19 mm]. However, the equalization parameters above are for a static test. A vehicle operating at track speed will not be as limber; therefore, track twist must be restricted. The allowable twist is usually expressed either as a percentage as noted above or as a ratio y:x. with y being an amount of superelevation and x being the length over which it is achieved, using the same units for both. A common limit is 1:400 as in 1 inch of superelevation in 400 inches/33.33 feet [roughly 25 mm in 10 meters]. However, some low-floor vehicle manufacturers have requested 1:500 as a track twist design limit. One U.S. transit agency that was having problems with center truck derailments on their partial low-floor LRV has established a maintenance standard of approximately 1:425. These ratios sharply contrast with the capabilities of legacy rolling stock with more limber truck designs. The PCC car, which was deliberately designed to operate on abysmal track, can deal with track twist of about 1:150. More to the point, the new twist limit figures are more restrictive than one of the formulas that has traditionally been used for determining minimum spiral lengths for LRT. That topic is discussed at length in Chapter 3, Article 3.2.5; however, the point to be made here is that track designers should obtain specific information from their peers on the vehicle side of the project regarding acceptable values of track twist. Ideally, the vehicle designers should provide three figures: â¢ A desirable twist ratio for track design. â¢ A minimum twist ratio for track maintenance. (This would be somewhat less restrictive and indicate the point at which corrective track surfacing should be undertaken.) â¢ An absolute minimum twist ratio to be used as a safety limit. This value, which may be speed dependent, would indicate that possible derailment is imminent unless corrective actions (either resurfacing of the track, speed reductions, or both) are taken. âJump frogsâ as described in Chapter 6, Article 6.6.6, are becoming a popular item for seldom- used diverging movements at special trackwork and were once very common on legacy streetcar lines. These will raise one wheel of the truck a dimension equal to the height of the wheel flange, typically 1 inch [25 mm]. Operation over the diverging side of such frogs must be done at very slow speed so that the vehicle suspension system has time to respond to the truck equalization
Light Rail Transit Vehicles 2-29 requirements. If jump frogs are proposed on an LRT project, that fact should be clearly identified in the vehicle procurement documents. 2.4.6 Light Rail Vehicle Ride Quality Light rail vehicle ride quality is defined in typical North American specifications as the capability to operate, at any speed up to the vehicleâs maximum operating speed (MOS) and at any passenger loading, free from vibration and shocks, to the specified levels. 188.8.131.52 Vehicle Natural Frequency as a Factor in Ride Comfort All of the light rail vehicleâs equipment is required to be free from resonance. To achieve this, resonances must be damped, and the natural resonance frequencies of the carbody must be sufficiently removed from the secondary suspension resonance frequency. Most vehicle specifications include language such as the following: The carbody natural frequency shall be 2.5 times the secondary suspension natural frequency. Vehicle specifications usually require that a dynamic and ride quality model should be developed using programs such as NUCARS or VAMPIRE and performance be proven via model predictions. The ride quality is evaluated according to ISO 2631, Mechanical vibration and shockâEvaluation of human exposure to whole-body vibrationâPart 1: General requirements, Figures 2a-Vertical and 3a Horizontal. In this case, the appropriate limit is the 8-hour fatigue limit to which the transit vehicle operator might be exposed. Transit patrons can be exposed to higher limits, as their exposure time would be considerably shorter. Note that the vehicle operator could be exposed to higher levels of vibration at the nose of the car than the patron would be at the center of the car. The ride quality is tested with a vehicle in good operating condition, with new wheels on tangent track that has been maintained to a class appropriate for the test speed, at vehicle crush loading of AW3. For this condition, the accelerations experienced by the passenger should generally not exceed 0.315 m/sec2 [about 1.0 ft/sec2], which is equal to 0.03 g. Another test, with air suspension deflated, is performed to confirm safe train operations under a partial failure condition and should not exceed 0.620 m/sec2 [about 2.0 ft/sec2] or 0.06 g. ISO 2631 does not specify specific test procedures. In the case of DMUs procured for one project, the tests were performed according to a European standard: UIC 518, Test and Acceptance of Railway Vehicles from the Point of View of Dynamic Behavior, Safety Against Derailment, Track Fatigue, and Quality of Ride. This standard determines vehicle compliance considering track alignment design, track geometry, and related operating conditions such as the cant deficiency and speed. 184.108.40.206 Track Geometrics as a Factor in Ride Comfort See Chapter 3, Article 3.2.4 for an extensive discussion concerning ride comfort as a factor in determination of characteristics of curved track, including speed, radius, superelevation, and spiral length.
Track Design Handbook for Light Rail Transit, Second Edition 2-30 2.5 VEHICLE STRUCTURAL LOADS 2.5.1 Static Vertical Loads ASME RT-1 defines light rail vehicle weights as follows: â¢ AW0: Empty load: the weight of the vehicle ready to run with all mounted components, including full operating reserves of lubricants, windshield fluid, etc., but without crew and passenger load. â¢ AW1: Fully seated load: AW0 plus the crew and fully seated passenger load. â¢ AW2: System load: AW1 plus 4 passengers per meter2 [3.3 per yd2] in standing areas. â¢ AW3: Crush load: AW1 plus 6 passengers per meter2 [5.0 per yd2] in standing areas. â¢ AW4: Structural load: AW1 plus 8 passengers per meter2 [6.7 per yd2] in standing areas. The mass of each passenger and crew member is stipulated as being 70 kg [154 lb], a figure that seems low at first glance, but makes allowances for children as well as adults of various statures. The AW4 loading is an extraordinary condition used only for the design of undertrack structures. 2.5.2 Wheel Loading Tolerance (Car Level) While most light rail vehicles appear to be completely symmetrical at first glance, the arrangement of various parts of the underfloor and rooftop equipment means that the actual loads applied to each truck will vary. A typical vehicle specification includes the tolerances related to overall weight distribution between the three or more trucks and the maximum acceptable wheel load variation per truck basis. While the numbers will vary, the following text is typical of the language found in vehicle procurement specifications for a three-truck articulated vehicle: â¢ The vehicle weight supported at center truck shall be within the range of 25 to 30% of the total vehicle weight â¢ The difference in vehicle weight between the A end and the B end trucks shall not exceed 450 kg (1000 lb) â¢ The lateral imbalance (wheel to wheel at the same axle, and expressed as a moment rotating vertically about the center of the axle) shall not exceed 100 kg-m (8500 in-lb) 2.5.3 Wheel Loading at Maximum Stationary Superelevation Worst-case wheel/rail force is expected when a fully loaded (AW3) car stops on a maximum superelevated track structure. Car tilt will also add to the lateral and vertical forces on the lower rail. The vehicleâs center of gravity projection when stationary on the maximum superelevation must be within the gauge of the tracks with a sufficient margin of safety. Typical practice is to keep it within the middle third of the track gauge; see Chapter 3, Article 220.127.116.11. 2.5.4 Unsprung Mass Unsprung weight in the LRV trucks is a significant contributing factor to dynamic track loading and ground-borne vibration as these items are not isolated from the track by the vehicleâs primary and
Light Rail Transit Vehicles 2-31 secondary suspension systems. The use of resilient wheels theoretically reduces unsprung mass to only the weight of the tire; however, the elastomeric elements of resilient wheels still need to be fairly stiff so as to keep the tire both circular and concentric with the axle. Hence, until relatively recent times, the axle and the gearbox were effectively unsprung mass. Modern truck designs achieve further isolation of the traction motor and gearbox unit by resiliently installing them on the truck frame and having the axle floating in the gearboxâs hollow output shaft, relying on a flexible coupling (âdog bonesâ) to transmit torque to the wheel set. The resilient wheel reduces truck shock and vibration, which is generally beneficial, but does introduce a resonance of the wheel set within the tire with a frequency of about 50 to 100Hz. The interaction among track stiffness, tire, wheel set, and truck frame is quite complicated and may vary considerably with design. This can be important with respect to track vibration isolation design. 2.5.5 Truck Design Light rail vehicle truck design has evolved appreciably since the light rail renaissance of the 1990s. The trucks on those early vehicles incorporated many features that had been successfully employed on heavy rail metro vehiclesâsuch as monomotor design (i.e., both axles powered by a single motor, rather than one motor per axle)âthat proved to be ill-suited for light rail vehicles operating on very sharp radius curves. Current designs build on that experience and provide much better performance (including a significant margin of safety against derailment) due to the following features: â¢ Shorter wheelbase (spacing between axles), which generally facilitates curving but can increase the angle of attack in a curve. (All other things being equal, a longer wheelbase truck will require wider flangeways and wider track gauge than a truck with a short wheelbase.) â¢ Longitudinally resilient axle mountings/primary suspension with resilient metal inserts. â¢ Resilient axle mounts in the transverse direction to reduce the impact upon entering the curve. â¢ Reduced unsprung massesâresilient wheels and drive units. â¢ Very low turning resistance due to being connected to the carbody with a ball bearing slewing ring and king pin without side plates. 18.104.22.168 Motorized Trucks Since the late 1990s, conventional power trucks have almost exclusively used AC traction motors and parallel helical gear units. These replaced the DC monomotors and hypoid gears commonly used on light rail vehicles up through the early 1990s. Figure 2.5.1 illustrates a typical power truck such as might be used under either a high-floor LRV or a 70% low-floor LRV. Features shown include AC motors and parallel gear units that are fully suspended resiliently on the truck frame, resilient wheels, chevron primary and air spring secondary suspensions, center king pin connection to carbody underframe, disk brake installed on the gear exit shaft, track brakes, train- to-wayside and cab signaling antennas, and on-board wheel flange lubrication.
Track Design Handbook for Light Rail Transit, Second Edition 2-32 The power trucks beneath 100% low-floor cars are much more sophisticated since they require room for the low-floor passenger cabin to pass between the wheels and truck frame. Figure 2.5.2 illustrates an outside frame truck design for narrow gauge track with the motors mounted longitudinally outboard of and between the wheels. The design powers both wheels on each side of the truck from a single motor, appreciably changing the way the truck interacts with the track compared with a conventional solid axle power truck. Figure 2.5.3 illustrates a low-floor power truck with conventional solid axles. This design utilizes small diameter wheelsâ600 mm [23.6 inches], roughly 100 to 110 mm [about 4 to 4.5 inches] smaller than the wheels used on most LRV trucks. The carbuilder also places the floor in the articulation module higher than the floor in the main body sections, with a ramp between the areas. Figures 2.5.2 through 2.5.3 are only a few of the many designs of low-floor power trucks that are on the market as of 2010. Some other designs utilize even more radical features such as individual âhub-mountedâ motors on each wheel. The state of the art is advancing rapidly and truck designs such as those illustrated here may well become obsolete. The reader is encouraged to review current trade publications and literature available on manufacturersâ websites for up-to-date information specific to the vehicles under consideration for a project. Figure 2.5.1 Kinki Sharyo power truck for 70% LRV
Light Rail Transit Vehicles 2-33 Figure 2.5.2 Siemens power truck for a Combino 100% low-floor narrow gauge LRV Figure 2.5.3 Bombardier Flexity Outlook power truck for 100% low-floor LRV
Track Design Handbook for Light Rail Transit, Second Edition 2-34 Figure 2.5.4 Kinki Sharyo trailer truck for 70% low-floor LRV 22.214.171.124 Non-Motorized (Trailer) Trucks Non-motorized trucks are typically located under the articulation joints of LRVs. On low-floor cars, the trailer trucks are located under the center section and donât rotate relative to carbody. They will not have motors and gear units, but will usually have braking systems. Because of their reduced mass, plus the configuration of the LRV carbody with respect to the trucks, the non- powered trucks frequently have lower axle loads than the powered trucks and hence apply less loading to the track. On high-floor cars, they will closely resemble the power trucks with the exception that they typically donât have motors, but the axles rotate, thus promoting steering. On low-floor cars, the non-powered trucks will have appreciably different designs than the powered trucks on the same LRV. In almost all cases of low-floor center section vehicles, there will be no rotating axle and each of the four wheels will rotate independently of the others. Figure 2.5.4 illustrates a typical trailer truck used under 70% LRVs in several North American cities. It is equipped with the same resilient wheels, primary and secondary suspension, and track brakes as the power trucks on the same cars. Disk friction brakes are located outside the wheels. The wheels are of the independently rotating (IRWs) type and are installed at the end of the low profile crank axle. Figure 2.5.5 illustrates an axle assembly for a truck with independently rotating wheels. Note the configuration of the cranked axle, permitting the low-floor to pass between the wheels, and the position of the roller bearing races interior to the hub of each wheel.
Light Rail Transit Vehicles 2-35 Figure 2.5.5 Kinki Sharyo cranked axle for low-floor LRV trailer truck 126.96.36.199 Load Leveling Both motorized trucks and trailer trucks typically include air bags as the secondary suspension. Leveling valves installed on the bolster sense changes in pressure between the air bags due to increases or decreases in the passenger loads and automatically inflate or deflate the air bags to restore the car floor level at the predetermined location in compliance with ADAAG. The adjustment necessary to compensate for the maximum of 1 inch [25 mm] loss of height due to wheel wear is accomplished by shimming under the primary suspension components, typically with rubber chevron springs. The accuracy of this type of adjustment is demonstrated during the vehicle acceptance tests. The orifice for the air access in the air bag is calibrated to provide the necessary damping precluding resonance. Additional rotary dampers are installed between the bolster and the truck frame. The carbuilder and the vehicle maintenance organization are largely responsible for ensuring compliance with ADAAG vertical tolerances for matching the elevation of the LRV door thresholds with the station platforms. This includes both the accuracy of car-leveling systems that compensate for variable passenger loading and the periodic insertion of shims in the truck assemblies so as to compensate for wheel tread wear. Vertical rail head wear is typically not accommodated by vehicle shimming as the amount of rail wear can vary significantly from station to station, particularly on a large and mature LRT network. Instead, the track maintainers will be charged with raising the track. Direct fixation track can be shimmed, and ballasted track can be raised. Embedded trackforms usually cannot be raised, and rail replacement might be necessary.
Track Design Handbook for Light Rail Transit, Second Edition 2-36 188.8.131.52 Inboard versus Outboard Bearing Trucks In its simplest form, a truck has two axles that are held parallel to each other by a truck frame. The points at which the frame is supported by the axles are called bearings. Typically, the bearings consist of a box enclosing roller bearing rings inside which the axles rotate. These bearing boxes can be located outboard of the wheels, on extensions of the axles that go beyond the outer face of the wheels, or the bearing boxes can be located inboard of the wheels. The majority of modern LRVs have trucks with inboard bearings, allowing easy access for replacement of the tires on resilient wheels without disassembling the bearings. The overall truck weight is also reduced since the axles are shorter. While outboard bearings are used on some standard gauge truck designs, they are more often found on trucks for tramways using narrow gauge track. A byproduct of the use of inboard bearings on a conventional solid axle truck is a reversal of the bending moments in the axles compared to an outboard bearings design. With outboard bearings, the moment loading on the axle between a bearing and the adjacent wheel creates tensile forces in the top of the axle and compressive forces in the bottom of the axle. Those forces are counteracted by the weight of the gearboxes, disk brakes, and other axle-mounted equipment so as to somewhat equalize stress in the axle. With inboard bearings, the moments are reversed as are the relative stresses in the axle. However, since the axle is rotating in both cases, these stresses are constantly cycling, setting the stage for possible metal fatigue. In either case, the axles must be designed to accept the stresses from the imposed loads and the cyclic reversal of loadings. However, since the axles are usually the heaviest single element within a conventional truck and since they are largely unsprung mass (with the exception of the minor cushioning provided by resilient wheels), carbuilders have made great efforts to reduce the mass of the axles to the minimum consistent with accepting the service loads within the appropriate factors of safety. Reducing the mass of the axles also reduces the amount of energy necessary to propel the LRV, which can have measurable life cycle cost ramifications. For this reason, many vehicle procurement specifications stipulate a maximum weight for the vehicle and include financial incentive/disincentive clauses for meeting or exceeding the goal. Where the track design gets into this issue is how the lateral loads from curving are applied to the track by the wheels. With inboard bearings, the lateral forces between the wheels and the outer rail of the curve result in a moment that tends to counteract the other applied moments and actually reduce stress in the axle. A possible problem arises when the track design incorporates restraining rails adjacent to the inside rail of the curve which, by design, share some portion of the lateral load with the outer rail. Any force between the restraining rail and the back of the wheels creates a moment in the wheel and axle assembly that increases the magnitude of the cyclic stresses in the axle. Because of this, many carbuilders and vehicle engineers stipulate that contact should never occur between the back side of a wheel and a restraining rail unless derailment is imminent, such as when the outer wheel has already begun to climb the outer rail. Exacerbating this situation is the fact that some resilient wheels are not designed to effectively transmit lateral forces applied against the back face of the tire. As discussed in Chapter 4, the use of restraining rail is a recommended practice with a long history of successful use in North America. However, most European track designers make comparatively little use of restraining rails (âcheck railsâ as they are called overseas) and instead
Light Rail Transit Vehicles 2-37 rely on the contact between the outer rail and wheel to accept all curving forces. Therefore, European carbuilders and other international carbuilders schooled in European practice do not typically expect there will be any force acting against the back of the wheel from a restraining rail. BOStrab, the German Federal standard regulations for tramways, actually prohibits routine continuous contact between the back of the wheel and any part of the track structure. Because of this fundamental difference in design philosophy, if the track design on a project includes restraining rails, that fact must be identified to the vehicle engineers at an early date and clearly explained in the vehicle procurement documents. The carbuilder will likely resist the use of restraining rails since it could require him to use heavier axles, increasing the unsprung mass and overall vehicle weight and possibly triggering a contract disincentive clause. The track engineer must therefore be prepared to strongly defend the use of restraining rails. See Chapter 4, Article 4.3.5, for additional discussion of this issue. 2.5.6 Vehicle DynamicsâPropulsion and Braking Forces The following parameters establish the maximum forces along the direction of the rails. The amount of adhesion is the measure of the force generated between the rail and wheel before slipping. A typical 4.8 kilometer per hour per second (3 miles per hour per second) acceleration rate is equivalent to a 15% adhesion level, if all axles are motorized. For a typical LRV with four of six axles motorized, the adhesion rate is 22.5%, which may have some bearing on rail corrugation rate and wear. Increased wear and corrugation rate suggest using hardened rail in acceleration zones and on grades. 184.108.40.206 Tolerances All acceleration and deceleration values also have tolerances that are due to many factors. The major factors for acceleration tolerance are traction motor tolerances, actual wheel diameter size, and generation and interpretation of master controller commands. This tolerance may range from Â±5 to 7%. All actual deceleration values are dependent on friction coefficients as well as the above issues. The expected tolerance for friction and track brakes should be obtained from the supplier. 220.127.116.11 Maximum Train Size Acceleration and deceleration forces are applied by all cars in a consist. Therefore, the total rail force per train will depend on the maximum train consist length. If more than one train can be on common rails at one time, this should also be considered. The tractive forces at the wheel/rail contact are independent of the number of cars for self-propelled cars under normal operation. More than one train in a track segment of interest is generally unlikely unless one train was inoperative and being towed or pushed by the other. In that circumstance, the inoperative train would be free-rolling (no power and no brakes) and would hence not apply any tractive effort to the rails. The pushing train might well be up at the limits of adhesion because of the drawbar forces, but that would be no different than the ordinary design criteria. Acceleration/deceleration rates would likely be less for trains with inoperative cars. Slip-slide control will also limit tractive contact forces in non-emergency situations.
Track Design Handbook for Light Rail Transit, Second Edition 2-38 18.104.22.168 Load Weight If the LRV has a load weight function, the acceleration and deceleration forces will be increased at car loadings above AW0 to some maximum loading value. These values should be defined to establish maximum longitudinal track force. 22.214.171.124 Sanding Car sanders apply sand to the head of the rail in front of the wheel to obtain a higher adhesion coefficient. Sanding in specific locations has a fouling effect on track ballast that should be considered. Sand will also accumulate in flangeways and special trackwork in embedded track. If the wheel/rail interface is over-lubricatedâa condition that makes use of sand more likelyâthe gummy mixture of sand and grease can become a significant housekeeping issue. Sanding may also have a detrimental effect on rail wear. 126.96.36.199 Vehicle Procurement Documents The procurement documents for light rail vehicles will very often include appendices intended to illustrate the service conditions under which the LRVs must be able to operate. Quite often, this will include plan and profile drawings showing the right-of-way characteristics, including the location of stations, curves, grades, and civil speed limits. If the LRVs are being purchased for an existing route, those parameters will be known exactly. In the case of vehicles for a new LRT line, the preliminary track alignment drawings will often be used as the best available information. The transit agencyâs manual of design criteria is often also included. In addition, the vehicle specification will stipulate the required vehicle performance characteristics and conditions under which the vehicle must operate, such as: â¢ Maximum acceleration, typically 3 mphps [1.34 m/s2]. â¢ Normal service braking rate (typically the same as maximum acceleration). â¢ Minimum emergency deceleration, typically 4.5 mphps [2.01 m/s2] considering a wheel/rail adhesion of 0.5. Higher levels of adhesion may raise the emergency deceleration rate to over 6 mphps [2.68 m/s2]. â¢ The most demanding service requirements, including routing between terminals, desired schedule speed, distances between station stops, dwell time at stops, passenger loadings, etc. â¢ Nominal line voltage and maximum line current. The LRV manufacturerâs design team will then determine the equipment and systems necessary for the cars to achieve the specified performance over the route. 188.8.131.52 Braking Forces Maximum braking forces during deceleration are determined for each track section based on grades and curves and are obtained with a combination of dynamic or regenerative braking (traction motor operating as generator), friction braking, and track brakesâall depending on the available adhesion. A contribution to the longitudinal forces and adhesion controlling is obtained with the load controlling system, sanding system, and slip-slide control system.
Light Rail Transit Vehicles 2-39 The following formula is a sample computation of the longitudinal force (F) on the track created by a three-car train during emergency braking and using a 0.5 adhesion coefficient leading to a deceleration rate (d) of = 3 m/s2 [6.74 mphps] at an AW3 load of 58,000 kg [about 128,000 pounds] per vehicle. M = 3 cars x 58,000kg/car =174,000 kg F = M Ã d = 174,000/9.81 x 3 = 53,211 kg [117,464 lb] 2.5.7 Dynamic Vertical Determination of total track force is a complex issue that depends on LRV design features. Typically the vehicle total weight is increased by a factor to include dynamic loading effects. The characteristics of the LRV suspension system should be defined by the manufacturer, who should also provide the dynamic load factor to the track designer. 184.108.40.206 Primary Suspension Primary suspension provides support and damping between the truck frame and the axle journal bearings. It is the first level of support and vibration control for the bearings above the wheel set. 220.127.116.11.1 Spring Rate Spring rate is the force per deflection of the coil or chevron primary springs. This relationship may be non-linear for long travel distances. The equivalent vertical, longitudinal, and lateral spring rates will generally be different. Chevron spring suspensions have high longitudinal stiffness, and the solid axles of trucks so equipped turn less easily through curves in response to rolling radius differentials. The longitudinal stiffness should be considered in track curve and rail head profile design. 18.104.22.168.2 Damping The damping is the âshock absorberâ action that provides a force proportional to the velocity of the spring movement. It is designed to minimize oscillation of the springs/mass system at the primary and suspension resonance frequency. 22.214.171.124 Secondary Suspension Secondary suspension supports the carbody on the truck and controls the range of carbody movement with relation to the truck. The suspension and track alignment basically establish the LRV ride quality. The secondary springs can be either steel coils or air bags. 126.96.36.199.1 Damping Damping is optimized for ride quality. With an air bag system, orifices in the air supply to the air bags can adjust the damping. 188.8.131.52.2 Yaw Friction Yaw is the amount of rotation of the truck about a vertical axis with relation to the carbody. With the exception of vehicles that have trucks semi-rigidly attached to a carbody segment (e.g., the Skoda-Inekon streetcar and others), yaw angles as high as 10 to 15 degrees occur routinely along sharply curved track. The truck design and materials used will establish the friction force that restrains truck yaw. High levels of yaw friction contribute to lateral track forces, which can
Track Design Handbook for Light Rail Transit, Second Edition 2-40 produce conditions where the wheel climbs over the rail head. The design of related friction surfaces should be such that the friction factor remains constant as service life increases. 184.108.40.206 Maximum Operating Speed The operating speed limit for all track considers passenger comfort and safety. This criterion should be coordinated with the car design. Civil speed limits for curved track are set by determining the maximum rate of lateral acceleration that passengers can comfortably endure. This is usually in the range of 0.1 g to 0.15 g, which establishes the level of unbalanced superelevation on curves. Speed limits on curves are then established based on the actual and unbalanced superelevation. See Chapter 3, Article 3.2.6, for additional discussion on maximum speeds in curves. Typically, there are no civil speed limits for tangent track other than arbitrary limits due to the characteristics of the trackway and vehicle. Therefore, the maximum speed on tangent track is typically determined by the vehicle mechanical limits, the train control system, and operating rules. The primary suspension stiffness will determine a stability speed limit that could be quite low. 220.127.116.11 Car Natural Frequency Light rail vehicles will have a natural frequency that should be considered during the design of civil structures such as bridges or elevated guideways. Trucks and car bodies each have different natural frequencies that should also be considered. Also, car loaded weight affects the carbodyâs natural frequency. Therefore, the vehicleâs natural frequency should be defined at the vehicleâs weight extremes, AW0 and AW3. (AW4 is not considered here since it is a theoretical loading only for design of bridges and virtually certain to never be experienced in service.) If the LRT system already exists and is being extended, there is likely an existing vehicle with natural frequency characteristics that will govern the design of structures. Conversely, if new vehicles are being procured for an existing system, the harmonic characteristics of the existing guideway should be considered in the vehicle procurement specifications. In particular, the bent passage frequency of a car traversing an elevated structure should not be coincident with the carâs secondary suspension resonance frequency. 2.6 TRACK GAUGE, WHEEL GAUGE, AND WHEEL CONTOURS Track gauge, wheel gauge, and wheel contours are some of the most important issues in the relationship between the light rail vehicle and the track. Each of these factors can vary appreciably depending on the characteristics of the light rail system. They are also a dynamic condition due to unavoidable wear of the wheel and rail running surfaces. There are three broad categories in which an LRT system might be placed, each with different ramifications for the track gauge, wheel gauge, and wheel contours: â¢ An existing or legacy system that has been in operation for many years and already has established standards for gauges and wheels. Presuming that performance is satisfactory, changing any of those parameters should only be undertaken with extreme caution after detailed investigation.
Light Rail Transit Vehicles 2-41 â¢ A new system that will share part or all of its tracks with a freight railroad operation. In such cases, there is usually very little opportunity to change anything, and it may be necessary to default to Association of American Railroads (AAR) and AREMA standards. â¢ A new system that will be an exclusive operation and have no interaction with freight railroad rolling stock. In this situation, both the trackwork engineer and the vehicle engineer have appreciable latitude to adopt track and wheel gauge and wheel contour standards that can optimize performance and minimize maintenance requirements. Performance in any of the categories above can be significantly affected by vehicle maintenance issues. If the maintenance plan and budget for the system does not provide for routine wheel truing, the track design may have to accommodate poor curving performance, higher impact forces, and more robust rail support to avoid adverse wear due to poor vehicle maintenance. 2.6.1 Track Gauge The American Railway Engineering and Maintenance-of-Way Association (AREMA) standard track gauge is established at 56 Â½ inches [1,435 millimeters], measured at 5/8 inch [15.9 mm] below the top of rail. While some light rail systems in North America that evolved from legacy streetcar lines use broad gauge track and no small number of European tramways use narrow gauge track, new light rail transit systems worldwide generally adopt standard railroad track gauge. The use of standard gauge track generally facilitates procurement of track materials and track maintenance equipment, although caution is necessary if circumstances result in wheel gauge different than railroad standards. For additional information on track gauge refer to Chapter 4. 2.6.2 Vehicle Wheel Gauge Vehicle wheel gauge (the distance between defined points on the face of the wheel flange) is always less than track gauge by some freeplay dimension. This is a very important interface issue that must be addressed jointly by vehicle and track designers. Failure to coordinate this issue can lead to interface problems with very costly long-term consequences. This is particularly important if the system will utilize embedded track using groove rails with narrow flangeways. Several LRT systems constructed in the 1980s through 2000 employed AAR standards for wheel contours and gauges, but also employed European groove rails. This resulted in routine interference between the backs of the wheels and the tram of the groove rail, reducing the service life of both. Standard wheel gauge for railroad cars per AREMA Portfolio Plan basic number 793 is established at 55 11/16 inches [1,414.5 millimeters]. However, that dimension, being specified to an arbitrary point on a compound curved surface, is very difficult to measure accurately, particularly as the wheels wear. A more convenient place to measure is between the inside faces of the wheelsâa dimension known as the âback-to-back distance,â often abbreviated as âB2B.â The back-to-back distance for AAR 1B narrow flange wheel sets mounted in accordance with AAR rules is 53 3/8 inches [1,355.7 millimeters]. This wheel mounting practice results in 13/16 inch [20.6 mm] of freeplay between track gauge and wheel gauge. This relatively large dimension is
Track Design Handbook for Light Rail Transit, Second Edition 2-42 necessary in railroad work because the acceptable maintenance tolerances for both track and wheel mounting are relatively large. In contrast, rail transit fleet sizes and track miles are both much smaller than they are for railroads, and it is somewhat easier to achieve tighter maintenance tolerances. In addition, for any rail system operating embedded track in city streets, smaller values of freeplay allow for narrower flangeway widths. Because of these factors, it has long been customary for street railway systems to employ smaller values of track gauge/wheel gauge freeplay than railroads. The former American Transit Engineering Association (ATEA), which set standards for both streetcar rolling stock and streetcar track in the first half of the 20th century, recommended that freeplay be set at Â¼ inch [6.4 mm], which is 7/16 inch [11.1 mm] less than AAR practice. This reduced freeplay dimension, coupled with the wheel contours recommended by ATEA, resulted in a back-to-back gauge of 54 inches [1372 mm] or more. Legacy systems that still use wheel gauge dimensions based on ATEA practices and any new LRT lines that adopt wheel contours and gauges that differ from AAR practice need to be very careful when procuring new equipment to be certain that their wheel gauge standards are understood by the manufacturers. This is often an issue when procuring maintenance-of-way equipment. Because of the narrow flangeways provided by most European groove rail sections, LRT systems that employ groove rail in embedded track will generally need to adopt a back-to-back wheel gauge that is wider than the AAR standard. The alternative is to either use one of the few groove rail sections that are specifically designed for use with railroad equipment or to narrow the track gauge to something less than standard. Wide groove rails are generally discouraged because even if they comply with ADAAG maximum dimensions for flangeways they are sufficiently wide that the mobility-impaired and bicycling communities will generally object to their use. Narrowed track gauge may be a practical option in tangent track, but may not be viable in curves and is generally not recommended. A secondary benefit of narrowed freeplay is reduced amplitude of any truck hunting. However, if conformal wheel contour is also used, a very small amount of movement might still result in a sufficiently large rolling radius differential to initiate self-centering and possibly hunting. A drawback of smaller values of freeplay between wheel gauge and track gauge is that, assuming tapered wheels, the maximum possible rolling radius differential is reduced. This means that solid axle trucks employing âtransit gaugeâ standard will begin flanging through curves at a higher radius than wheel sets conforming to railroad practice. However, large clearances between wheel and track gauge allows a higher angle of attack at curves, exacerbating flanging. This is not much of an issue on many rail transit lines as their average curve radius is often well below the threshold at which flanging occurs. Track maintenance standards for tight track gauge must be more restrictive, with reduced freeplay, and a minus tolerance of zero is recommended. Track gauge narrowing has been specifically employed at small radius curves to reduce the angle of attack and thus noise and gauge face wear. In any case, no gauge widening should be employed at any curve on transit systems, as such will promote high angle of attack. While gauge widening is common in the United States, such practice hails from the days of three-axle locomotive trucks.
Light Rail Transit Vehicles 2-43 TCRP Report 71: Track-Related ResearchâVolume 3: Exothermic Welding of Heavy Electrical Cables to Rail, Applicability of AREMA Track Recommended Practices for Transit Agencies (prepared under TCRP Project D-7) addresses many issues relevant to the interface between LRT track and LRV wheel sets that are not covered by AREMA. It is strongly recommended that the users of this Handbook also consult TCRP Report 71. 2.6.3 Wheel Profiles Wheel profile is one of the most critical vehicle parameters to consider in track design, since the wheel is the primary interface between the vehicle and the track structure. The wheel profile must be compatible with the rail section(s); the special trackwork components, including switch points and frog flangeways or moveable point sections; the guard rail positions to protect special trackwork components; and restraining rail if used on sharp radius curves. Once accepted, any changes to the wheel profile (especially tread and flange width) must be evaluated by both vehicle and track designers. In more than one instance, the wheel profile has been altered at the last minute by the vehicle side of a project without informing the track designer, resulting in unsatisfactory performance of both the track and vehicle. The first edition of the Track Design Handbook for Light Rail Transit (also known as TCRP Report 57) illustrated a dozen different wheel contours that were in use on North American light rail lines at the time. The differences were startling, and there was seemingly no consistency. Several designs had their origins in AAR practice, while others could be traced back to ATEA designs. Still others resembled wheels used on some European railway systems, and their selection may have been influenced by the overseas suppliers of the LRVs and/or track materials. Looking at those wheel designs in light of current understanding of rail/wheel mechanics, only two or three have sufficient merit to warrant consideration for any new light rail rolling stock. Rather than possibly misleading readers into thinking all those wheel designs are all recommended designs, they have been omitted from this second edition in favor of discussions of characteristics that can be found in a good wheel design. Parties with an interest in some of these other wheel contours can consult TCRP Report 57 for additional information, although it must be understood that some systems may have changed their wheel contour since TCRP Report 57 was published. 18.104.22.168 AAR-1B Wheel Contour The Association of American Railroads (AAR) promulgates two standards for wheel contours on rolling stock. The AAR-1B wide flange contour is generally of no interest to transit work. The AAR-1B narrow flange contour is used on locomotives, railroad passenger cars, and some freight equipment. Both versions of the AAR 1B wheel were adopted as their standards during the 1990s, replacing much older designs that had been AARâs standards since the 1920s. AAR-1B wheels incorporate a compound curve radius at the throat between the flange and the wheel tread. This is designed to conform to similar radii on the heads of AREMA standard rail sections. This conformal contact facilitates curving by maximizing the rolling radius differential between wheels on the same axle and also promotes self-centering of wheel sets in tangent track. The conformal contact at curves may also reduce contact stresses and thus wear. The AARâs former wheel design, which is still used by several LRT systems, has a single radius in the throat. The wheel profile is considered to be conformed to the rail profile if the gap between the
Track Design Handbook for Light Rail Transit, Second Edition 2-44 wheel and rail profile is less than 0.5 millimeters [0.02 inches] at the center of the rail (in single- point contact) or at the gauge corner (in two-point contact). Both the old and current AAR wheel designs incorporate a 1:20 taper on the wheel tread so as to facilitate truck centering on tangent track and self steering on slight curves. The AAR-1B wheel profile is an evolution from a design first proposed by Professor Herman Heumann (1878â1967), a German railway engineer who did pioneering work in the field of wheel- rail contact mechanics. Some elements of Professor Heumannâs work have been superseded by subsequent research (notably his endorsement of a 70-degree flange angle), but that is the result of better analytical methods and changes in the demands placed on the rail wheel interface rather than any flaws in his theories. Tests by the AAR at the Transportation Test Center in Pueblo, Colorado, have shown that the AAR-1B wheel profile provides â¢ A lower lateral-over-vertical (L/V) load ratio in a 764-foot [233-meter] radius curve than the previous AAR non-conformal wheel. â¢ A lower rolling resistance than the previous AAR profile. Arguably, this is less important in a transit vehicle, which might have 66% or even 100% of its axles powered, versus a locomotive-hauled freight train, which might have only 5% of the axles powered, but it does have some ramifications for life cycle energy and maintenance costs. â¢ Lower critical hunting speeds than the old AAR wheel profile. This means that, all other things being equal, trucks equipped with the AAR-1B wheel will commence hunting at a lower speed than the AARâs old non-conformal wheel. The hunting speed is primarily a function of wheel tread taper at the center of the tread running surface. The last bullet point is significant, and some discussion is appropriate. âHuntingâ is the tendency of a wheel set with tapered wheels to uncontrollably oscillate from flange to flange while seeking to center on the track with a consistent rolling radius on each wheel. This is a dynamic condition, highly sensitive to the natural frequency of the truck design as well as the presence or absence of dampers (e.g., shock absorbers) to control truck rotation (yaw). With a conformal wheel, compared to a wheel having either a straight taper leading to a small flange/tread radius (or even no taper in the case of a cylindrical wheel), a smaller amount of lateral movement is required to create an appreciable difference in rolling radius, thereby initiating self-centering action. Overcompensation could then initiate hunting behavior at certain speeds. Informal observations suggest that âworn wheelâ designs similar to the AAR-1Bâwhich was designed for relatively large values of gauge freeplay per freight railroad standardsâmay on some vehicles and truck designs hunt excessively when freeplay is tightened down to transit standards. This is likely due to running closer to the flange throat, where the taper becomes large. The overall system needs to be proportioned so that with the wheel set centered on tangent track there will be no routine contact between the gauge corner radius in the wheel flange throat and the crown radius of the rail head. This is an area that requires additional research. Wheel tread wear will tend to reduce the taper from the new condition. In the extreme case, when maintenance intervals are too long or wheel truing is simply non-existent, excessive wear of the wheel will produce a âfalse flangeââa relatively unworn zone on the outside of the wheel
Light Rail Transit Vehicles 2-45 tread that lies below the plane of the top of rail. On the field side of the concave worn tread, the wheel taper will actually be negative. Such worn wheels are often referred to as having a âhollow tread profile.â Poor curving performance will occur, with potentially poor performance on tangents, contributing to rail corrugation and wear. 22.214.171.124 Transit Wheel Design and Selection While shared track with a freight railroad operation might force the selection of the AAR-1B narrow flange wheel and AAR wheel gauge, most new LRT operations have more latitude in selecting an optimal wheel profile. Rail car designers have several computer programs available that enable them to model the dynamic characteristics of the vehicle, including the behavior of the proposed wheel profile for a given trackform and variations in rail head shape, gauge freeplay, and other factors. Examples include NUCARS, AdamsRail, and VAMPIRE. Figure 2.6.1 illustrates a wheel contour that has been successfully employed on a U.S. LRT system that uses both 115RE tee rail and 51R1 groove rail. It could be considered as a starting point for determination of the optimal wheel for a new LRT system without railroad interface. Figure 4.2.2 in Chapter 4 illustrates the same wheel superimposed on the track and illustrating gauge and freeplay issues. Since the time when this wheel was developed, the dimensions of 115RE rail have been revised to incorporate an 8-inch [300 mm] crown radius, hence this wheel profile may no longer be optimal. Figure 2.6.1 Candidate initial LRV wheel profile (All dimensions in inches)
Track Design Handbook for Light Rail Transit, Second Edition 2-46 The paragraphs that follow describe some of the issues that must be considered when selecting or developing a wheel profile for light rail transit. 126.96.36.199.1 Tread Conicity Wheel treads virtually always have a conical taper when new (usually 1:20) so as to promote self- centering in tangent track and some degree of steering in flat curves. Conical/tapered wheels have been common since the early 20th century. However, a very few legacy rail transit properties continue to use cylindrical wheels, having originally adopted them long ago to resolve problems with uncontrolled truck hunting. That solution came with the penalty of loss of self- centering and increased wear on rails and wheel flanges in curves. Cylindrical wheels also need more frequent maintenance to correct the development of false flanges. Better methods are available to control hunting today through truck design, so cylindrical wheels are not recommended. Some transit properties have adopted flatter or steeper tapers than 1:20 and/or use a steeper conicity outboard of the normal wheel/contact zone. The latter defers the need to do wheel truing to correct hollowing of the wheel tread, but, in general, frequent wheel truing is strongly recommended as part of a comprehensive preventative maintenance program. Some literature suggests that tapered wheels may promote wheel squeal at curve, due to a positive feedback effect as the wheel vibrates across the rail head. This behavior is theoretical, but may explain why wheel squeal appears to be more prevalent at rigid track than in poorly maintained track built with jointed rail that is only loosely fastened to the ties. This is a curious situation that deserves more investigation. 188.8.131.52.2 Tread Width The tread on AAR wheels is over 4 inches wide, that being necessary to ensure the wheel can reliably bridge the open throat of the intersecting flangeways in turnout frogs, given the relatively loose tolerances on railroad track gauge and wheel set maintenance. Transit systems, having a captive fleet and higher standards for track and wheel set maintenance, can generally employ narrower flangeways in frogs and proportionally narrower wheels. If the track system employs flange-bearing frogs throughout, the wheel tread can be very narrow as the wheel tread is not in contact with the frog through the open throat. Narrow wheel treads also reduce the unsprung mass of the wheels, with appreciable benefits concerning impact forces and energy consumption. Narrow tread wheels are typically combined with wider back-to-back wheel gauge, the reduced freeplay compensating for what might otherwise be a reduction in the available wheel/rail contact surfaces. See Article 2.6.6 for additional discussion on wheel tread width. 184.108.40.206.3 Flange Face Angle Older wheel designs, such as those recommended by the former ATEA, had relatively flat flange angles. An angle of 27 degrees to the vertical (63 degrees to the axle) was common. Research at the Transportation Technology Center, Inc. (TTCI), as documented in TCRP Report 71: Track- Related ResearchâVolume 5: Flange Climb Derailment Criteria and Wheel/Rail Profile Management and Maintenance Guidelines for Transit Operations, demonstrated with numerical simulations that wheel flanges positioned at an angle of 72 to 75 degrees with respect to the axle are much less likely to climb the rail than the old flatter flange angles. The factor that describes the propensity for a wheel to climb the rail is known as the Nadal Value. Wheels that comply with the old ATEA designs were found to have Nadal Values of about 0.70 to 0.75. By contrast, the
Light Rail Transit Vehicles 2-47 AAR-1B wheel and transit wheels of similar design have Nadal Values of about 1.1, indicating a much reduced tendency to climb the rail and hence a greater margin of safety against derailment. To be fully effective, the 75-degree flange angle should be constant (i.e., not part of a curved surface) for a distance not less than 0.1 inch [2.5 mm]. APTA adopted this standard as part of their recommended practice for commuter railroad equipment. As of 2010, APTA had not endorsed this feature for light rail and metro rail passenger equipment, but it can be safely asserted that it represents good practice. Many European tramways use wheels that have an even steeper flange face angle of 1:6 (about 80.5 degrees to the axle), which matches the gauge face slope that is common on European groove rail sections. 220.127.116.11.4 Flange/Tread Radius As noted above, nearly all modern wheels incorporate a conformal compound curve radius in the throat between the wheel tread and the flange. This should closely match the radii used on the gauge corners of the rails to be used on the LRT. Designers are cautioned against mixing different rail sections in the track design unless the selected sections present a reasonably consistent contact surface to the wheel. In that regard, it should be noted that many groove rail sections have gauge corner radii that are radically different from that of 115RE tee rail. 18.104.22.168.5 Flange Back Angle/Radius Most wheels, including the AAR-1B, have a relatively broad radius between the radius on the flange tip and the flat face of the back of the wheel. This eases the transition of the wheels into guarded special trackwork and is hence desirable for smooth operation. In the case of track systems that employ restraining rail, the angle of the back of the wheel should be carefully considered with respect to both the horizontal angle of attack between the wheel and the restraining rail and the vertical angle of the restraining rail. Three dimensional modeling of the contacting surfaces is suggested. 22.214.171.124.6 Flange Height The flange height is the vertical distance from the tip of the flange to a point on the wheel tread known as the taping line (see Article 126.96.36.199.9). Legacy streetcar lines, particularly those with flange-bearing special trackwork, often use very short wheel flanges. Three-quarters of an inch [19 mm] is common, which contrasts sharply with AAR wheel flanges that are 1 inch [25 mm] tall. Short flanges have several serious design issues: â¢ They are generally incompatible with the AREMA 5100 undercut switch point design because the tip of the wheel flange is above the top of the leading end of the switch point. On one LRT project, short flanges on legacy rolling stock that had worn even shorter in service would routinely climb the second cut on the top of the diverging switch point and derail. An aggressive wheel reprofiling program along with a wholesale modification of the stock rails was necessary to stabilize the situation. â¢ Their short height also provides a very narrow contact band with the gauge side of the rail when passing through curves, leading to accelerated gauge face wear on both the rails and the wheels. â¢ They provide virtually no height for the desirable minimum straight flange face angle when combined with a conformal compound radius in between the flange and the tread. For these reasons, the recommended minimum flange height is 1.0 inch [25 mm].
Track Design Handbook for Light Rail Transit, Second Edition 84-2 188.8.131.52.7 Flange Thickness Typically, the flange thicknessâthe horizontal dimension from the projected vertical back face of the wheel to the gauging point on the front of the flangeâshould be about 7/8 to 1 inch [22 to 25 mm]. This allows for a reasonable amount of flange face wear before wheel truing becomes essential. In general, wheel truing should not be deferred until the flange thickness reaches a condemning limit, since by then it might not be possible to restore the flange without removing an excessive amount of the wheel tread surface, substantially reducing the wheel diameter. Reduction of wheel diameter often triggers the need to shim the trucks so that the vertical relationship between the vehicle doorways and the platform remains in compliance with ADAAG. If the track design will use groove rails with extremely narrow flangeways (generally any flangeway less than about 1 Â½ inches [38 mm] wide), it will usually be necessary to reduce the flange thickness from the recommended dimension above. Such thin flanges will require more frequent wheel truing and are not recommended. 184.108.40.206.8 Flange Tip Shape The tips of the wheel flanges on systems that use flange-bearing special trackwork tend to wear flat or nearly so, slightly decreasing the height of the flange. To prevent this loss of height, the wheel flange for use with flange-bearing frogs should have a tip that is either flat or has a very broad radius for a width of at least Â¼ inch [6 mm] to reduce contact stresses. This then compounds into a shorter radius that blends into the angles on the front and back face of the flange. 220.127.116.11.9 Wheel Diameter LRV wheels are generally 24 to 28 inches [610 to 710 mm] in diameter. This measurement is made at a point on the tread that is a consistent distance from the back face of the wheel and nominally where the wheel tread contacts the top of the rail when the wheel set is centered on the track. It is known as the âtaping lineâ since that is the location where the circumference of the wheel is measured with a specially calibrated tape. The diameter of a wheel has a direct effect on the length of the âfootprintâ that the flange has at the top-of-rail level. This in turn affects how the wheel interacts with the rail, especially in curves and through special trackwork. The footprint of small diameter wheels could be less than the length of open frog throats and could present challenges with respect to providing proper guarding of the frog. See Chapter 4 for a discussion about the generation of Wharton diagrams and Nytram plots and for the determination of the most appropriate track gauge and flangeway widths for a given wheel. Mixed fleets that have more than one wheel diameter must consider each one independently, even if they all have the same wheel profile. 18.104.22.168 Independently Rotating Wheels (IRWs) Independently rotating wheels, having no solid axle to force paired wheels to have the same rotational velocity, behave appreciably differently in curved track. Curving behavior is modified, reducing longitudinal slip, but flange face wear is greater on IRWs than on the wheels of the power trucks on the same LRVs due to increased angle of attack. IRWs tend to produce more flanging noise than solid axle wheel sets, again due to increased angle of attack and lateral creep velocity. This issue was investigated in TCRP Project C-16, and informal observations that had
Light Rail Transit Vehicles 94-2 been made on several transit properties operating 70% low-floor cars were confirmed. As a result of this accelerated wear, it is generally necessary to reprofile IRWs more frequently and replace the resilient wheel tires more often than on solid axle wheel sets. 22.214.171.124 Miscellaneous Considerations for Wheel Contours 126.96.36.199.1 Historic Streetcars Several light rail transit systems have antique streetcars (or modern replicas of same) that are operated over the tracks of the system on either an occasional or scheduled basis. The wheels on such rolling stock must be considered to the same degree as those of the LRV fleet. In general, any such vehicles should be retrofitted with wheel contours conforming to the adopted standard for the system. Exceptions might be made for a one-time use, such as the opening day ceremonies for a new LRT system, provided the wheels on the heritage vehicle are in good condition and the back-to-back wheel gauge is consistent with the special trackwork. Badly worn wheels, particularly any which have short flanges or false flanges, should not be permitted Even if the heritage vehicles will be equipped with new wheels, some modifications may still be required in the event that the heritage vehicles have wheel diameters or truck wheelbases that are substantially different from the regular LRV operating fleet. Many pre-PCC vintage streetcars have wheel diameters that are appreciably different (both much larger and much smaller) than those of modern LRVs. These differences directly affect the footprint of the wheel flange at the top of rail elevation. Such wheels should be evaluated closely using Filkins-Wharton diagrams and the Nytram plots as discussed in Chapter 4. 188.8.131.52.2 Shared Trackage with Freight Railroad In the event that the LRT shares track with freight trains, special trackwork that conforms to AREMA standards for flangeways and check gauge and adoption of the AAR-1B wheel (or something close to it) will usually be essential. However, if the LRT system also includes embedded track sections using narrow flangeway groove rails, it may be necessary to both employ a compromise wheel contour and modify the special trackwork in the shared-use area. Such combined systems became popular in Europe during the 2000s, following the success of a pioneering âtram-trainâ operation in Karlsruhe, Germany. Such systems typically use ordinary tramway tracks in downtown areas and switch onto local or regional freight railroad tracks in suburban or interurban areas. Compatibility is achieved by both using a modified wheel, as seen in Figure 2.6.2, and providing elevated guard rails opposite frogs in the shared track areas. In Figure 2.6.2, the 7.5 mm [0.30 inch] projection on the back face of the wheel provides a back- to-back distance that complies with European practice on freight railways while the back-to-back gauge at the wheel tread elevation complies with transit practice. The overall width of the wheel provides for safe operation over railroad frogs while the outer taper provides assurance that the wheel tread overhang will not initially contact the pavement in groove rail areas. (Some contact may occur as the rail wears and would need to be corrected by pavement grinding.) Wheel gauge and gauge freeplay match transit practice and present no problem on well-maintained freight track.
Track Design Handbook for Light Rail Transit, Second Edition 2-50 Figure 2.6.2 Compromise wheel for Karlsruhe tram-train (all dimensions in millimeters) No tram-train systems have been constructed in North America, although DMU operations in southern New Jersey and Austin, Texas, have some tram-train characteristics. There are institutional issues related to the regulations of the Federal Railroad Administration that make it somewhat unlikely that tram-train technology can be fully applied in the United States. That situation notwithstanding, the Karlsruhe wheel is illustrative of what can be possible when trackwork and vehicle designers collaborate to achieve a desired goal. 184.108.40.206 Average Worn Wheel Conditions Chapter 2 of the first edition of the Track Design Handbook for Light Rail Transit included an extensive discussion of investigations made concerning interactions between trackwork and badly worn, âhollowedâ wheels with pronounced âfalse flangesâ on the outer edges of the wheels. That discussion originated in research done for freight rail operations and generally has no applicability to a light rail transit system that performs routine wheel truing as part of a comprehensive preventative maintenance program. The focus of investigations into wheel/rail interactions is generally on the performance of new wheels running on new rail, a condition that exists only briefly on any project. Arguably, the condition of most interest is the behavior of the system with both rail and wheels âworn in,â but well before either reaches a condemning limit. Wheels generally wear much faster than rails. So some investigation about the performance of average worn wheels running on average worn rails might be appropriate. For an operating system with little maintenance budget, the track designer may be faced with accommodating a variation of tread profiles for the same vehicle. All of these options are appropriate for wheels and rails in good condition as well. Designing for the worst- case profile is appropriate, and close coordination between track and vehicle maintenance providers is necessary in any case.
Light Rail Transit Vehicles 2-51 2.6.4 Maintenance of the Wheel/Rail Interface When the first edition of the Track Design Handbook for Light Rail Transit was published, there had been relatively little investigation into the rail/wheel interaction under transit vehicle loadings. Since that time, there has been a good deal of investigation under the auspices of TCRP Project D-7, with the results published as a series of volumes collectively known as TCRP Report 71. As of 2011, the D-7 project is ongoing (and is expected to continue indefinitely), providing factual information specifically targeted at rail transit instead of conjectural extrapolations of the results of research done under freight railroad loading. Rail transit system maintenance procedures have come under increased scrutiny since 2000. As of this writing, the states are responsible for oversight of the process, but federal oversight is increasing. Partially in response to this regulatory scrutiny, APTA has developed recommended practices for transit rail car maintenance, including wheels.,  Most rail transit systems are now following system-specific wheel management procedures, consistent with the APTA guidelines, with respect to inspection and maintenance of wheels including truing of worn wheels. New Jersey Transit has developed comprehensive standards for wheel maintenance that could be considered a model program. This program includes the following standards: â¢ Wheel maintenance procedures are included in the System Safety Program as a mandatory requirement. â¢ Wheel wear conditions are checked with either a digital output hand-held profile gauge or on the truing machine as part of a mandatory daily vehicle inspection. â¢ Wheel reprofiling is performed either at fixed intervalsâevery 30,000 to 40,000 miles [48280 to 64374 km] depending on the truck designâor as periodic measurements indicate the need for corrective action. â¢ Intermediate wheel profiles are used as determined by software incorporated in the wheel truing machines. As many as 20 variants of corrective actions are recommended by the machine so as to minimize the removal of metal from the wheels. With this program in place, New Jersey Transit has increased resilient wheel tire life dramatically, typically achieving 200,000 to 250,000 miles [322,000 to 402,000 km] of service before tire replacement is necessary. Maintenance of the track side of the wheel/rail interface, principally through a comprehensive program of rail grinding and strategic lubrication, is equally important. See Chapters 9 and 14 for discussions of these topics. 2.6.5 Matching Wheel and Rail Profiles Since wheels are a machined item and finished on a lathe, it is relatively easy to procure customized wheel contour designs to suit particular applications. The same flexibility is not available in the selection of rail profiles since rails are finished on a rolling mill. Further, of the roughly two dozen rail sections commonly available, only a very few are actually suitable for use by rail transit. However, wheel and rail profiles must be compatible, which generally means that the wheel profile needs to be detailed to conform to the as-rolled head profile of the selected rail.
Track Design Handbook for Light Rail Transit, Second Edition 2-52 As with wheel profiles, the majority of the research and development work regarding rail head profiles and rail profile grinding has been undertaken by and for the railroad industry. While the transit industry can also benefit from this research, readers are cautioned that recommendations for heavy haul railroads are very often less than entirely applicable to the transit industry. The difference in maximum wheel load between a light rail vehicle and a fully loaded freight car can be a factor of 4 or 5. Because of this large difference, rails used in transit service will not be subjected to wheel forces of the magnitude exerted by freight cars. Therefore, theories of rail gauge corner fatigue, high L/V ratios, and the threat of rail rollover that pertain to freight railroads are generally less applicable on a transit system. To illustrate the differences between conformal and non-conformal wheels, Figure 2.6.3 illustrates the 115 RE rail section used on contemporary LRT systems in conjunction with both the obsolete AAR wheel profile and the newer AAR-1B wheel profile. Note how the non-conformal two-point contact wheel/rail relationship of the non-conformal wheel transfers the vertical load from the gauge corner toward the centerline of the rail. This combination reduces the wheel radius at the contact location, which is detrimental to steering and introduces accelerated gauge face wear. In practice, the wheel gauge corner will tend to wear to the rail and vice versa, developing some modest conformal contact over the long term. However, as the system matures, normal maintenance will result in the introduction of new and freshly reprofiled wheels and replacement of worn rail with new rail, resulting in inconsistent wheel/rail contact. A mixture of rails and/or rail cant conditions on a single system will result in non-uniform rail profiles at the gauge corner and tend to frustrate achieving a systemwide stable gauge corner profile for the worn wheel. To improve wheel/rail interface contact on older systems, alternate wheel shapes may be considered. During the early design stage of new transit systems, transit wheel profiles should be considered that match or conform to the rail section(s) to be used on the system. In the process of wheel design, the design engineer must consider both the rail section(s) and the rail cant at which they will be fastened. For additional information on rail sections, refer to Chapter 5 of this Handbook. For additional information on rail cant selection and benefits, refer to Chapter 4, Article 4.2.5. Figure 2.6.3 Wheel-rail interface
Light Rail Transit Vehicles 35-2 2.6.6 Wheel Tread Widths and Flangeways at Frogs When a wheel passes through a frog, the wheel tread must pass over the open throat of the intersecting flangeway. In an ordinary (not flange-bearing) frog, the load on the wheel will briefly transfer from the inner to the outer part of the wheel tread and then back again as the wheel passes over this gap. For this transfer to be smooth, the wheel tread must be appreciably wider than is required to support the wheel in ordinary track. See Chapter 6, Figures 6.6.1 and 6.6.2 for an illustration of how a wheel traverses a frog. The large value of freeplay between AAR wheel gauge and standard track gauge requires a wider flangeway opening through frogs and guard rail flangeways than when following transit standards. The wider flangeways allow larger lateral wheel movement, resulting in less wheel tread contact if the wheel set has shifted furthest from the gauge face of a frog point. If the wheel tread is too narrow, this condition results in hammering of the wing rail and the frog point due to insufficient tread support when the wheel transfers between the two components. Narrow wheels traversing the frog in a facing point direction lose the wing rail wheel support too early, resulting in premature transfer of wheel load to the narrowest portion of the frog point, resulting in batter and crushing of the frog point. In a trailing point orientation, the batter occurs on the wing rail instead of the frog point. To minimize these problems, the AAR standard wheel has an overall width of 23/32 inches [145.3 mm]. 5 A wider wheel tread increases the weight of the wheel, thereby increasing the unsprung mass of the truck and impact forces by a small but measurable amount. Wide wheels can also abrade adjoining pavement in embedded track areas. A narrower overall wheel width is therefore desirable. The suggested minimum width for a transit system that shares its track with freight cars and hence needs to follow AREMA-recommended practices for flangeway widths, is 5 Â¼ inches [133 millimeters]. This dimension includes a Â¼-inch [6-millimeter] radius at the field side of the wheel tread. Wheels that are narrower cannot be used with railroad standard flangeways and wheel gauges as doing so will lead to improper wheel traverse through special trackwork components. Reduction of both flangeway widths and wheel widths is possible in special trackwork that does not need to deal with freight equipment, particularly if transit gauge freeplay standards are followed. 2.7 RESILIENT WHEELS Nearly all North American LRVs use resilient wheels such as the Bochum Bo54, Bochum Bo84, SAB, and the Acousta-Flex wheel designs. A few other designs are also in use. Resilient wheels have a long history of use on rail vehicles as a means of reducing the impacts between the rail and the vehicle. The earliest resilient wheels actually appeared in the late 19th century, using compressed paper as the cushioning element in the wheels beneath railroad sleeping cars. Several experimental designs of resilient wheels existed for streetcars in the 1920s, but the first large-scale use of cushioned wheels occurred with the introduction of the PCC streetcar in the mid-1930s. The PCC resilient wheels (there were several variations) were of the âsandwichâ design, with the compressed rubber components oriented in the plane of the wheel and hence in shear under loading. Such wheels could handle a maximum vertical
Track Design Handbook for Light Rail Transit, Second Edition 2-54 wheel load of approximately 6000 pounds [about 2700 kg], which was sufficient under the relatively light PCC car. Heavier cars required more robust resilient wheel designs than could be managed with a sandwich design. One of the more popular designs was the Bochum Bo54 wheel, introduced in 1954, which placed a series of rubber blocks in compression between a wheel hub and outer ring-shaped tire. The Bo54 design worked well, but required sophisticated equipment (âThe Bochum Pressâ) to change the tires. In response to that issue, the Bochum Bo84 design made tire replacement much easier and cost-effective. Bo84 resilient wheels were designed to withstand a vertical wheel load of 12,000 pounds [5,443 kg]. Other designs based on the same principles are available from international vendors, many of whom have licensed U.S. firms to manufacture their products. Ignoring heritage streetcars, there are extremely few light rail vehicles that still utilize solid wheels. The advantages of resilient wheels compared with solid steel wheels are â¢ Noise reduction/attenuation due to the rubberâs absorption of structure-borne vibrations. One study revealed a reduction of noise of 25 to 30 dBA for resilient wheels versus solid wheels. Resilient wheels are particularly effective in reducing sustained wheel squeal at curves, probably due to damping and the ability of the tire to deflect about a vertical axis through the contact patch. However, flanging noise is not reduced, though it is generally of much lower amplitude than sustained wheel squeal from solid wheels. â¢ Decrease of wheel and track wear due to the rubber blocks placed between the tire and the hub. One study suggests that flange face wear is half what it would be for solid wheels. This has distinct advantages with respect to wheel truing since, when wheels are turned, most of the reduction in wheel diameter is not to remove defects in the wheel tread but rather to restore the thickness of the wheel flange. â¢ Reduction of unsprung mass to the weight of the tire. By contrast, in a truck with solid steel wheels, the entire mass of the wheels and axle is unsprung. â¢ Resilient wheel tires are available with better material properties than those of rigid wheels. The typical resilient wheel tire has a hardness of 320 to 360 BHN compared with solid wheels, which have a hardness of 255 to 290 BHN. The harder wheel is hence closer to the strength of heat-treated premium rail. Softer wheels would have been sacrificial to the rail when it comes to wear. The harder wheels are closer to parity. â¢ Reduced wheel set shock and vibration, which is beneficial to trucks with rigid couplings between the axe and gear box out shaft. Brake discs mounted on the axle also benefit from reduced shock and vibration. The rubber springs of both the Bo84 wheel and Bo54 wheel are mounted in compression for vertical loads and act in shear for lateral loads. The lateral stiffness of the Bo54 and Bo84 wheels is controlled by providing a chevron-shaped cross section, which is incorporated into the Bo84 wheel as shown in Figure 2.7.1. Lateral shift of the tire relative to the hub of the wheel is thereby significantly reduced. Modern resilient wheel designs have also increased the allowable tread wear, and tire replacement can now be performed without truck removal. Higher loadings are
Light Rail Transit Vehicles 55-2 now possible without overstressing the wheel. One vendor reports commonly handling lateral forces of up to 45 kN [10,000 lb] with a vertical load of 60kN [13,500 lb] with no reported failures or problems. Figure 2.7.1 illustrates Bo84 wheels as used by New Jersey Transit. The larger wheel tire on the left uses an AAR-1B wheel profile as well as AAR back-to-back wheel gauge and freeplay and is used on NJTâs Hudson-Bergen LRT line. The smaller wheel is used on NJTâs Newark City Subway routes and accommodates a back-to-back wheel gauge of 54.125 inches [1375 mm] and a reduced value of freeplay. While the same light rail vehicle is used on both routes, a different wheel is required on the Newark City Subway routes because they evolved from a legacy streetcar system. For additional information on resilient wheels, see Chapter 9, Article 220.127.116.11. 2.8 ON-BOARD VEHICLE WHEEL/RAIL LUBRICATION As is discussed in Chapter 9 of this Handbook, lubrication of the wheel-rail interface is a proven method of reducing wheel squeal noise. A simple observation of this can be made on any rainy day, when merely a thin film of water dramatically reduces wheel squeal. Traditionally, the application of lubricants and friction modifiers to the rails has been a responsibility of the track maintenance department. However, maintenance of trackside lubrication equipment has always been difficult and proper operation therefore erratic. Common problems include either too much or too little product applied and too little of it finding its way to the point of need. In addition, application of friction modifiers in embedded track areas can cause safety issues with motor vehicle traffic and pedestrians. Because of these issues, placing the lubrication equipment on the light rail vehicle is very attractive. It brings the equipment to the vehicle maintainer for servicing instead of requiring the track maintainer to go to multiple equipment sites, making maintenance and resupply more likely to occur. It also provides an opportunity to better control the application rate. However, the Figure 2.7.1 Bo84 wheels used by NJ Transit
Track Design Handbook for Light Rail Transit, Second Edition 2-56 initial method of on-board lubrication, solid stick lubricators held by spring pressure against the flange of the wheel, have generally been unsatisfactory. Several situations have changed that collectively show promise of creating an optimal method of getting friction modifiers to the locations that most need it: â¢ Better lubricants and friction modifiers that are vastly superior to and more environmentally friendly than common mineral oils and greases. These products have better characteristics for friction values, adhesive power, corrosion protection, and phase separation. They are also stable independent of temperature and can be sprayed. See Chapter 9 for additional information. â¢ Reliable spray equipment designed to match these new products that can be mounted on light rail vehicles. â¢ Global positioning system (GPS) technology that enables automatic activation of the on- board equipment at curves and other locations requiring the friction modifier without demanding action by the vehicle operator. As of 2010, approximately a half-dozen rail transit agencies in North America have adopted on- board spray equipment for targeted application of wheel flange and top-of-rail friction modifiers. This system shows both good results (such as control of wheel squeal to less than 80 dBA) and great promise for being a maintainable technology. 2.9 VEHICLES AND STATIONSâADA REQUIREMENTS The Americans with Disabilities Act (ADA) requires that public operators of light rail transit systems make their transportation services, facilities, and communication systems accessible to persons with disabilities. New vehicles and construction of facilities must provide the needed accessibility in accordance with the ADA Accessibility Guidelines (ADAAG). As a guideline, new light rail transit stations should be designed taking into consideration the ultimate ADA goal of providing universal access for persons with disabilities. The track alignment designer may need to consider the following when setting the track horizontal and vertical alignment. â¢ Horizontally, the ADAAG requires providing platform edges that are within 3 inches [75 millimeters] of the edge of the vehicle floor with the door in the open position. Some LRVs have thresholds that project beyond the face of the vehicle so that the clearance between the platform and the carbody may legitimately be in excess of the ADAAG dimension. â¢ Persons entering a building normally expect a slight step upward, not down, and expect to be stepping down when exiting. Because of this human nature factor, it is important that the vehicle floor never be below the platform. Therefore, the vehicle floor elevation should generally be slightly higher than the station platform elevation so that disembarking patrons have a slight step down.
Light Rail Transit Vehicles 2-57 To properly address ADAAG requirements, designers will consider all dimensional tolerances of the platform/vehicle interface, such as â¢ Track-to-platform clearances. â¢ Vehicle-to-track clearances. â¢ Vehicle dimensional tolerances, new/worn. â¢ Vehicle load leveling. The tight horizontal and vertical clearance requirements between the vehicle door threshold and the platform edge impact the construction of track. To maintain these tolerances, some properties have used rigid trackforms to structurally connect the track and the platform. Others seek to only deter ballasted track from lateral movement toward the platform by using extra length crossties butted against the platform foundation wall. 2.10 REFERENCES  New Jersey Transit/PB, Crashworthiness Study, October 1995.  NJ Transit, Specification for Light Rail VehiclesâDecember 1995.  NJ Transit Low-floor Light Rail CarâA Modern Design, TRB-APTA Joint LRT Conference. Dallas, TX, 2000.  NJ Transit/ Kinki Sharyo, Proposed Increased Capacity LRV with a 5-Section Articulated Vehicle Using Existing Vehicle Modules, 2009.  General Order 143-B, Safety Rules and Regulations Governing Light Rail Transit, Title 6, Construction Requirements for Light Rail Vehicles, Public Utilities Commission of the State of California (revised January 20, 2000).  EN 15227/2008, Railway applicationsâCrashworthiness requirements for railway vehicle bodies.  EN 12663/2000, Railway applicationsâStructural requirements of railway vehicle bodies.  ASME RT-1, Safety Standard for Structural Requirements for Light Rail Vehicles, 2010.  NJ Transit, LRV Specification- As Built, Contract 96CT001, October 2006.  North American Light Rail Vehicles 2008âA Booz-Allen Compendium.  APTA SS-M-015-06, Standard for Wheel Flange Angle for Passenger Equipment.  APTA RP-M-009-98, Recommended Practice for New Truck Design.  ISO 2631-1:1997 (E), Mechanical vibration and shockâEvaluation of human exposure to whole-body vibrationâPart 1: General requirements.  UIC 518, Test and Acceptance of Railway Vehicles from the Point of View of Dynamic Behavior, Safety against Derailment, Track Fatigue, and Quality of Ride.
Track Design Handbook for Light Rail Transit, Second Edition 85-2  Wu, H., X. Shu, and N. Wilson, TCRP Report 71: Track-Related ResearchâVolume 5: Flange Climb Derailment Criteria and Wheel /Rail Profile Management and Maintenance Guideline for Transit Operations, Transportation Research Board of the National Academies, Washington, DC, 2005.  Griffen, T., TCRP Report 114: Center Truck Performance on Low-Floor Light Rail Vehicles, Transportation Research Board of the National Academies, Washington, DC, 2006.  49 CFR 659, Rail Fixed Guideway Systems, State Safety Oversight.  Kalousek, Joe & Magel, Eric, Managing Rail Resources, AREA Volume 98, Bulletin 760, May 1997.