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34 Shared Use of Railroad Infrastructure with Noncompliant Public Transit Rail Vehicles: A Practitioner's Guide other methods of limiting potential damage from a derailment should be considered. Detectors generally don't prevent problems, but provide early warning of hazardous conditions, and can be more easily installed. When a detector is triggered, an interface with the train control system normally causes nearby signals to display a STOP aspect and also broadcast a radio alert to oper- ators and the control center. Generally the operating rules stipulate that all traffic must halt until a restart movement command is received from the control center. The control center would investigate the cause of the alarm and confirm a safe condition exists before authorizing traffic to resume. Interoperability of Freight Trains in Shared Territory A key issue for shared-track operations is the compatibility of freight equipment with the train control system. The locomotives used on branch lines can be a dedicated fleet, and fitted with the vehicle-borne cab signal apparatus. They will operate normally in a cotemporaneous man- ner, with the train control system ensuring safe spatial separation between trains. The controls on board a locomotive would be programmed differently to factor in freight braking rates and operating speeds. Freight train lengths may exceed the length of light passenger rail trains. A longer freight train could occupy two or more track blocks. The train control system should automatically accommodate the longer train. The most difficult condition is the operation of a nonequipped "foreign" (i.e., from a railroad outside the shared-track corridor) locomotive on the shared-territory. Where unequipped freight equipment operates, special operating Rules and Procedures will assure operating safety. · Temporal separation methods can be used to backstop the occasional situations where non- equipped conventional equipment must detour over the shared territory. · If conventional trains only travel for a short distance on shared-track, turnouts can be set and locked to give the nonequipped freight train exclusive possession while they make the move- ment. The train control system will still prevent intrusions by passenger trains. Application of speed control systems to freight trains is potentially problematic. Enforced braking of freight trains has two principal drawbacks. 1. The braking performance of a freight train is more variable and unpredictable, depending on the mix of loaded and empty cars, the car types in the train, and the condition of the brakes. 2. Too high a braking rate can result in high longitudinal forces in the train, potentially leading to damage and derailment. However, with a properly designed enforcement system, freight locomotives could apply a low braking rate that does not pose a derailment risk. This is feasible and may be justified for a captive fleet of locomotives where freight trains travel for a substantial distance on the shared-track. Planners are advised to review typical freight cargo, train lengths, operating speeds, active tracks, car counts, and other traffic characteristics along with track alignment and geometry. This can aid in identifying potential hazards or warning system locations. 9) Fail-Safe Train Separation If a true cotemporaneous operation is planned, a higher level of safety assurance is required. The train control system must prevent a human error or component failure from jeopardizing operational safety. A minimum off-the-shelf shared-track train control system should feature intermittent or continuous cab signaling. The recommended system uses appropriate combina-
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Enabling Shared-Track: Technology, Command, and Control 35 tions of traditional signaling technology. The system should incorporate automatic train stop capability and overspeed protection. · Automatic train stop capability: vehicle-mounted components that communicate with the signal system to force a train stop when there is insufficient separation from the train or obstruction ahead. The primary additional device is a receiver to read and process informa- tion carried by track circuits, beacons, or electro-magnets. The carborne receiver is specific to the chosen train control system. · Overspeed protection: limit train speed to below posted speed limits to prevent excessive speeds on curves or through switches and to provide adequate stopping distance. In general, these methods of providing fail-safe train separation supply additional function- ality using elements physically integral to the rest of the basic train control system (track circuits, line wires, and bungalows and wayside signals) and adding appropriate components. Essentially, the train control system monitors train speeds where speed reduction is necessary. The location to begin braking is selected on the basis of entry speed, deceleration rate, and dis- tance to go before encountering a stop signal or a collision hazard. a) Applications of Automatic Train Stop Systems (ATS) (Intermittent) Basic railroad-style ATS systems are unlikely to provide an adequate level of collision safety, especially on single-track lines. The high number of train meets at passing sidings, and the chance of an "acknowledge and forget" event constitutes unacceptable risk. Transit-style ATS, with an enforced stop on passing a signal at danger combined with modest top speeds and high braking rates is an option for less demanding applications. Transit ATS could be applied where freight traffic is very low or where passenger vehicles are to be prevented from encroach- ing on a short section of track "locked out" temporarily for a freight movement, as on NJ Transit's Newark City Subway. Enhanced ATS appears to be the most attractive for concurrent shared-track operations. The risk of "acknowledge and forget" incidents is much reduced by the additional warnings and enforcement to limit speed approaching a danger signal, and by enforced braking on passing the danger signal. b) Applications of Cab Signal Systems (Continuous) Traditional power frequency fixed block systems with cab signaling and enforcement, like that installed on the Northeast Corridor and on New York area commuter rail systems, is mature and also would be suitable. Designed for high density, high speed operation with mixed locomotive-hauled and multiple-unit trains, they will likely exceed safety require- ments for ordinary shared track operations. c) Audio Frequency Coded Track Circuits: The State of the Art Continuous cab signal technology (in "b" above) requires high capital and maintenance costs. Recent technological advances have reduced somewhat the life-cycle costs of coded-track cir- cuit based systems. Compared to a traditional cab signal system, the audio-frequency track circuit system has many advantages. · Lower capital and maintenance costs Testing and maintenance burden is minimized Fewer relays are used, thus bungalows and relay cases are smaller Fewer impedance bonds Fewer conductors means fewer terminations and less wire tagging, simpler installation Very few insulated joints, reduces installation and track maintenance costs · Easier to bid and award because of nature of technology and larger pool of vendors · Track circuit lengths are easier to tailor to a route Longer in line-haul segments Shorter near stations and crossings, for better operational control