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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
