Washington Metro signaling and operation

Signaling and operation on the Washington Metro system involves train control, station identification, train signaling, signage, and train length. As with any working railroad, communication between train operators, dispatchers, station personnel and passengers is critical. Failures will result in delays, accidents, and even fatalities. It is therefore important that a comprehensive signal system operated by a central authority be in place. This gives individual train and station operators the information they need to safely and efficiently perform their tasks.

Metro was designed to use an Automatic Train Control (ATC) system, which comprises three sub-systems. Automatic Train Protection (ATP) protects trains by separating them so as to avoid collision. Automatic Train Supervision (ATS) routes trains and assists in maintaining adherence to schedule. Together, these two sub-systems provide input to the train's cab signals and to a third sub-system, Automatic Train Operation (ATO), which can control the trains directly. Since a train collision in 2009 caused by an ATC malfunction, Metro did not use the ATC system following the accident until it was re-introduced in September 2014

The Automatic Train Protection sub-system uses coded track circuit technology originally supplied by Rochester, New York-based General Railway Signal when the line was constructed in the 1970s. It is a life-critical system that provides a continuous stream of information to the train regarding the maximum safe speed via the running rails. Speed commands are designed to ensure trains maintain a safe stopping distance from any obstruction and do not exceed the speed limit for any segment of track. The system stops trains in advance of any other train traffic, and before stop signals at interlockings. The cab signalling system relies on track circuits to detect track occupancy, and thus send the appropriate speed code. Although the ATP system was designed to be fail-safe, the track circuit control equipment on which it relies has been prone to parasitic oscillations which can cause the system intermittently to fail to detect the presence of a train. This resulted in the 2009 collision, after which Metro began to operate all trains manually. In September 2014, Metro began reintroducing its computerized control system after taking a number of measures to avoid accidents similar to the 2009 accident.

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