The replacement of an important piece of collision-prevention equipment five days before last week's fatal train crash on the Washington, D.C., Metro's Red Line did not prevent the tragedy, as the track circuit failed and the malfunction went undetected, according to investigators and transit officials. Metro rail chief Dave Kubicek said the circuit in question did not completely fail, but "fluttered" on and off so rapidly that Metro's downtown operations center would not have easily spotted it. The circuit system's failure casts doubt on the reliability of the highly automated subway system, which is supposed to be "fail-safe."
In fact, many parts of Metro's automated train control system still use original equipment that is based on 100-year-old relay technology. The rail system is split into blocks and is designed to keep at least two blocks of distance between trains to prevent a collision. Each block is equipped with at least one track circuit that detects the presence of a train using audio frequencies transmitted between the train and the rails. There are two Wee-Z bonds within the track circuits that register the presence of a train and automatically send signals to the next train down the line, and if the following train gets too close the Wee-Z bond transmits a "zero" speed signal that forces that train to halt.
National Transportation Safety Board investigators said the June 22 train crash, in which an idling train was struck from behind by another train, was caused by the Wee-Z bond's failure to detect the presence of the idling train. That means that the striking train did not receive a "zero" signal and its onboard systems would have been automatically set to travel at 59 mph. Kubicek said the Metro rail system is safe and called the circuit flutter an "anomaly."
From The Washington Post
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