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30 Shared Use of Railroad Infrastructure with Noncompliant Public Transit Rail Vehicles: A Practitioner's Guide relevant factors. All designs must consider failure scenarios involving train control technology, rail vehicle functions, and human factors. Most freight branch lines are dark (unsignaled), and therefore lack basic train protection capabilities. With low traffic levels operating at low speeds, train control mechanisms are sim- ple and inexpensive to maintain. If any passenger service is contemplated on such a route, addi- tional features need to be incorporated to provide better train protection, more operational safety, and flexibility. Certain features are mandated by regulation, whereas others simply improve the service or line-haul capacity. Although a passive system (dependent on the human operator) may be acceptable, an active system (that compensates for human error or component failure) is preferred. The addition of a train control system can be viewed as a no cost fringe benefit from the freight operator's perspective. Whatever the design requirements or features incorporated in a basic train control system, there are fundamental regulatory requirements for any train control system design: Prevent entry into an occupied block; Stop distance to signals must be based on full-service brake rate; Provide broken rail protection; and Invoke automatic train stop systems based on maximum authorized line speed. Each impacts the train control system design for shared-track operations. 3) Train Control Technology--Conventional Systems Conventional or traditional train control technology is based on fixed blocks, multiple aspect, power frequency or direct-current (DC) track circuits. The fundamental element of a basic train control system is the block. The block is a section of track with defined limits. Its occupancy is governed by a signal. Figure 2 shows a simple example. Each vertical tic mark indicates a separate block. Block lengths are established during signal design. Each block is an electrically separate track circuit, and individual lengths vary. Train movement is controlled by signals that require an appropriate response by the train operator. Train control technology now in service in the United States is mature, reliable, well under- stood and based on simple, time-tested principles. In fact, this advanced stage of development obstructs the introduction of new technology or its adaptation to nonconventional applications. 4) Train Control--Emerging Technology--PTC and CBTC Positive Train Control (PTC) and Communications Based Train Control (CBTC)were devel- oped to expand the train control and information services provided by the signaling system. PTC refers to a North American family of train control technologies that provides functionalities over and above the most advanced continuous cab signal systems. PTC and CBTC are being devel- oped for high-density or high-speed lines such as urban heavy rail rapid transit lines and mixed passenger/freight main line applications. However, they are not necessary for fail-safe train sep- aration required for concurrent operation of conventional railway rolling stock and light pas- senger rail cars on shared track. Figure 2. Signal block layout.