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The US National Transportation Safety Board (NTSB) announced last night that it had identified an apparent safety signal-system design anomaly in its investigation of the fatal Washington, DC Metro collision in June.

Since the crash, suspicion has centered on the track circuits which were found to exhibit anomalous behavior.

The NTSB announced that it ... 

"... has discovered that a  failure occurred in which a spurious signal generated by a track circuit module transmitter mimicked a valid signal and bypassed the rails via an unintended signal path. The spurious signal was sensed by the module receiver which resulted in the train not being detected when it stopped in the track circuit where the accident occurred."

Obviously, it is not good when the rail safety system thinks it has received a valid "track clear" signal when it should have received a "track occupied" signal.

The NTSB has made urgent, specific recommendations to the Washington Metropolitan Area Transit Authority (WMATA) and to Alstom Signaling, Inc., the manufacturer of the track circuit modules, to examine the WMATA track circuits and work together to eliminate adverse conditions that could affect the safe performance of these systems. It has also informed other federal transportation authorities to make sure the word goes out to other transit systems that use similar signaling systems to Washington's Metro.

You can read the NTSB letter to Alstom Signaling here which provides specific details about the identified problem.

NTSB Chairman Deborah Hersman said it was still early in its investigation but that "its findings so far indicate a pressing need to issue these recommendations to immediately address safety glitches we have found that could lead to another tragic accident on WMATA or another transit or rail system."

Major parts of the Los Angeles County Metropolitan Transportation Authority subway system use the same computer control system as the Metro, for instance.

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