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26 Shared Use of Railroad Infrastructure with Noncompliant Public Transit Rail Vehicles: A Practitioner's Guide Thus, equivalent safety can be expressed by the mathematical relationship that includes the chance of a hazard event, the measure of consequence, and the predetermined time period. While the mathematics is straightforward, the availability of real world accident and incident data may be limited. To demonstrate safety equivalence, a variety of operating and accident sce- narios must be modeled and tested. Assumptions of consequences may not be based on histor- ical data, and thus might be overly conservative or optimistic. The benefit of a risk assessment methodology is that a repeatable procedure and reproducible data is provided, ensuring objective scenario comparisons. However, the methodology also has some critical shortcomings: (1) lack of actual data for a specific scenario; (2) assumptions regard- ing consequences; and (3) limitations on the scenarios envisioned. A risk analysis template show- ing the process is contained in Part IV after the business case template. Underpinning the Case for Shared-Track The fulfillment of safety and regulatory objectives is focused on the features and characteris- tics of technology used to control the operation and provide the service. Most other institutional concerns are addressed via legal agreements, financial arrangements, memoranda of understand- ing or other official and formal commitments. But safety is a front line issue, where the rubber meets the road. Here, shared-track projects are most vulnerable to rejection, delay, modification or the addition of costly features and technology mandated by regulators. A risk analysis is a component of the safety case. And the safety case is essential to support the business case. It should be recognized that the choices in technology would impact the risk assessment, which in turn affects the costs and operations cited in the business case, the freight operation, FRA Waiver Petition content, and SSO approval. There are trade-offs to consider and decisions should not be made in isolation. The approach to assessing the merits uses the tools described in Chapter 4 "Shared-Track: A Handbook of Examples and Applications" to determine the choice. Business model--outlines participants and relationships. Business case--defines the fiduciary contributions cost/benefits of alternatives. Safety case--analyzes the relative safety of the alternatives, defines roles of FRA and SSO, and the influence of the risk assessment. One shortcoming of the risk analysis is that the theory and results are not fully understood or appreciated by sanctioning authorities. The probabilistic aspect does not satisfactorily address a nightmare scenario event. There is simply less comfort in calculating a one-in-a-billion chance of an accident event every 10 years. Regulators can more easily understand that if an event occurs, then passengers are protected.