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48 Shared Use of Railroad Infrastructure with Noncompliant Public Transit Rail Vehicles: A Practitioner's Guide equipment. These features provide a higher degree of safety for their operations. Their perform- ance and design characteristics provide an advantage at grade crossings (the number one location for accidents on commuter rail lines), not only to avoid many such collisions, but also to reduce hazards that result from collisions. Such factors should be considered in addition to train control systems and operating procedures when approval for shared-track operations is requested. In many cases, one of the DMU types will be the most appropriate vehicle choice, as low floor diesel powered vehicles are more easily adaptable to route changes, extensions, and pilot pro- grams than LRVs that require a wayside power source (e.g., OHC). Ultimately, the vehicle ought to be considered one part of an integrated system of safety that relies on crashworthiness, train control, communications, training, and R&P. The rail car com- ponent of this system should not be burdened unduly to mitigate all hazards. 1) Selecting the Optimal Vehicle Selecting a heavy or light EMU/DMU or LRV (light passenger rail cars) vehicle is primarily based on operating speed and propulsion system assumptions. Additional influences on vehicle selection and design include: Operating environment (railroad, on-street, grade separated or reserved ROW); Clearances (primarily width and height); car static and dynamic envelope; Platform interfaces; Weight restrictions; Future flexibility for service changes; and Wheel tread and flange profile. A suitable vehicle will likely operate across multiple environments in normal service. Where multiple types of right-of-way are used, operating restrictions, weight, turning radii, and clear- ances on any part of the line influence the technical specifications of rail vehicles. 2) Regulatory Approach Currently FRA's policy considers commingled operations adequate when accompanied by a pos- itive train separation system. The FRA may consider modifying regulations for such applications. For example, future regulations might offset structural strength requirements with collision energy management design. Regulations could be altered further to describe certain key operating and vehicle design characteristics more suitable for shared-track (e.g., specify freight speeds, minimum track centers, lateral clearances, train control system, energy absorbing features, deceleration rates), effectively creating a new tier of vehicles. Defining certain minimum performance characteristics and features would simplify the FRA's process of evaluating each vehicle and waiver petition. 3) Standardization If a standard light passenger rail car model could use modular components and systems to allow limited unique system modifications (e.g., to the capacity of an HVAC system) and alter- nate suppliers, such a model effectively could reduce capital and maintenance costs for all oper- ating agencies. An economic benefit of standardized vehicle designs is the resulting cost savings. While each rail car is different, this distribution of proportional costs is useful for planning or budgeting purposes. The data also emphasize three other benefits of standardized designs, car bodies, and systems. 1. Regulatory review process is eased because the FRA does not have to initiate a fresh review for each new light passenger rail car waiver petition. Standardization also results in more accumulated service history with a specific vehicle model. Although the latter is a noneco- nomic benefit, it may enhance the appeal of the concept.