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Enabling Shared-Track: Technology, Command, and Control 29 Example scenarios: A B Head to Head Train A is routed into siding Train B runs stop signal A Flanking Train A has a straight ahead move B Train B runs stop signal B A Rear-End Train A is delayed Train B runs stop signal Figure 1. Train accident scenarios. Train control technologies for railroad and transit operations are mature, but continue to evolve with the introduction of improved technology and components that offer additional capabilities. Train control systems are first and foremost installed to ensure safety. They provide three basic protective functions: 1. Train detection--indicates presence and location of trains; 2. Train separation--maintains safe following distances between trains; and 3. Route interlocking--prevents unsafe moves on/off branches or conflicting routes through crossovers and turnouts (that might cause collision or derailment). Conventional signal systems are required by federal regulations where passenger train speeds exceed 60 mph, although shared-track would merit a signal system at any speed. Above 80 mph, federal regulations generally require active protection against three situations regardless of oper- ator performance: (1) entrance to occupied block; (2) overspeed with respect to signal aspect; and (3) operator error. The most significant limitation for shared-track applications of conventional fixed block is that multi-aspect signal technology, typically sufficient for passenger operations (below 80 MPH), is not adequate for shared-track operations, due to its lack of active protection. Wayside signals relay information with the expectation that the operator will respond properly. Override capa- bilities are not provided to catch and correct operator error. Consequently such conventional signal systems are not likely to be deemed acceptable for a shared-track environment with light passenger rail cars, regardless of speed. 2) Train Control System Design Parameters The design of signal systems must be based on assumptions and parameters that include max- imum speed, train acceleration and deceleration rates, train length, route gradient, curves, and civil speed limits. Other factors, such as number of tracks and features like reverse running, also are considered. Designers usually apply various safety factors (for example, diminished braking performance and additional stop distance margins) to system criteria to allow for potential fail- ures or malfunctions of the vehicle. As shared-track operations are planned, the signal system must accommodate both short light rail cars and longer freight trains, with widely different stop- ping distances. Adjustments to basic designs are made to take into account system service objectives, protection features, overspeed conditions, wheelslip/slide conditions, brake system failures or deficiencies, gradient, curvature, visibility, civil speed limits, rail volumes and variety of traffic, and other