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On Tuesday, the National Transportation Safety Board (NTSB) issued nine safety recommendations — six marked urgent — to address concerns about the safety of train control systems featuring audio-frequency track circuits. The recommendations are the result of an ongoing investigation into the collision of two Washington Metropolitan Area Transit Authority (WMATA) Red Line trains in Washington, D.C., on June 22.

The NTSB so far has determined that a failure occurred when a “spurious signal” generated by a track circuit module transmitter “mimicked” a valid signal and bypassed the rails via an unintended signal path, according to a statement issued by the NTSB. A modular receiver sensed the spurious signal, causing a train to be undetected when it stopped in the track circuit where the accident occurred.

The NTSB has recommended that WMATA and track circuit module supplier Alstom Signaling Inc. examine the track circuits and work together to “eliminate adverse conditions that could affect the safe performance of these systems.” The NTSB also called on WMATA to develop a program aimed at periodically determining that electronic components in its train control systems are performing within design tolerances.

The NTSB recommends that the Federal Transit Administration (FTA) and the Federal Railroad Administration (FRA) advise all transit agencies and railroads that use such circuits in their train control systems about the preliminary findings in the WMATA accident investigation.

The board also recommends that the FTA and FRA require agencies and railroads to examine their track circuits and work with their signal equipment suppliers to eliminate conditions that “could affect the safe performance of these systems,” and develop programs aimed at periodically ensuring that electronic components in train control systems are performing within design tolerances.

“Our 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,” said NTSB Chairman Deborah Hersman.