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Enabling Shared-Track: Technology, Command, and Control 43 lished a temporal separation requirement for light rail equipment that operates in a shared-track environment because it lacks the required buff strength. The FRA did contemplate the possibil- ity of commingled operation under some form of fail-safe train separation and an affirmative risk analysis. However, no potential operator has fully undertaken the heavy burden of trying to meet this requirement. Recent studies have acknowledged that temporal separation would be the guiding principle for shared-track because any other service would require some form of waiver from certain FRA regulations, given that existing regulations do not allow latitude to dispense with FRA compli- ance. Nevertheless, the state-of-the-art in these vehicles provides a variety of improvements that may make it easier to prove equivalent safety to the FRA, or at least near-compliance, especially when the vehicle safety features are augmented with other wayside systems and train control technology that contribute to overall safety. Review of Suitable Candidate Rail Vehicles While many vehicles and propulsion systems can serve this potential market, it is most likely that a selected vehicle will be equipped with a diesel engine prime mover whose propulsion sys- tem is either electric or hydraulic. Diesel as the choice of primary power source is fundamentally driven by system cost issues, since diesel eliminates the expense of electrification and also offers greater route and service flexibility. And a conventional roadway or marine diesel engine that fulfills current EPA regulations can be used. Moreover, a diesel prime mover avoids potential electrical clearance limitations to freight traffic and associated signal system complexities. Many common terms of reference [such as diesel multiple units (DMU), light rail vehicles (LRV), electrical multiple units (EMU)] can be confusing or unclear to both experts and non- specialists. For report purposes the term "light passenger rail cars" is suggested as a generic refer- ence to all vehicles in service on, or considered for, shared-track operations that do not comply with FRA structural requirements (49 CFR Part 238). Where a specific reference (DMU, LRV, or EMU) is employed, it is used to focus the discussion on a particular subset of the universe of non- compliant vehicles under consideration. Appendix 7 is a "mini-catalogue" of typical light passen- ger rail vehicles, although some are more suited to shared-track than others. 1) Light Rail Vehicles LRVs suitable for the shared-track environment have evolved from vehicles typically used as streetcars. Such LRVs are currently in operation in a number of U.S. cities, such as San Francisco, Boston, and Philadelphia. LRVs constructed for shared-track do, however, differ from LRVs designed for operation in urbanized areas, in a number of ways. The shared-track LRVs tend to be longer, wider, and heavier than the vehicles designed strictly for operations in urbanized areas, and they operate at higher speeds. However, one of the recent main differences between standard street running LRVs and those intended for shared-track is the variety of propulsion methods, physical dimensions, and capability to enable these vehicles to operate on two or more different rail lines, including a downtown or street run- ning portion. 2) Diesel Multiple Units and Electrical Multiple Units DMUs and EMUs have been used traditionally to operate as commuter and intercity trains on lines with low ridership or those that require a high frequency of service. Those vehicles are con- structed much like standard railway coaches with the addition of a propulsion system and an operator's cab. A new generation of lighter DMUs and EMUs (jointly referred to as light passen- ger rail cars) has been designed with the shared-track market in mind. They resemble current