"The NTSB said that its ‘testing found that a spurious high-frequency modulated signal was being created by parasitic oscillation from the power output transistors in the track circuit module transmitter. This spurious signal propagated through the power transistor heat sink, through the metal rack structure, and through a shared power source into the associated module receiver, thus establishing an unintended signal path. The spurious signal mimicked a valid track circuit signal.'
" In separate letters to Metro, the FTA, the FRA and Alstom Signaling, Inc., the NTSB revealed that the track circuit equipment generated a false signal, which ‘resulted in the train not being detected when it stopped in the track circuit where the accident occurred.' The NTSB stated that the ‘signal mimicked a valid track circuit signal' and as a result the ‘ability of the track circuit to detect the train was lost.'
"The NTSB's recommendations could impact transit systems across the nation. The NTSB stated that it was ‘concerned about the safety of train control system circuitry used in comparable rail and transit operations in other parts of the country.'
"The NTSB has identified a symptom of the problem with the track circuit, but not a root cause or a solution," said Metro General Manager John Catoe. "We are doing everything we can to make our rail system as safe as possible. The NTSB still has not identified a probable cause for the accident. They made two recommendations and we will implement those recommendations. In fact, we began to do so weeks ago."
"The NTSB recommended that Metro examine its track circuits that may be susceptible to sending false signals to eliminate the problem and that Metro develop a program to periodically determine that electronic components in its train control system are performing within tolerances. (The NTSB issued similar recommendations to the Federal Transit Administration and the Federal Railroad Administration, which oversees transit agencies and railroads in the country that use similar track circuits.)
"Metro has already begun to implement the first recommendation by taking the following actions:
1. Metrorail trains are being operated manually by train operators and will continue in manual mode until further notice.
2. Metro began addressing this recommendation the week of the accident when it started running twice-daily computerized tests of all of its track circuits, once after each rush hour, or 14 times per week. This computerized analysis identifies similar events so that track maintenance personnel can make any needed adjustments to the track circuits. If the analysis identifies a problem, the area of track is operated in "absolute block," which means only one train at a time is permitted to be in a designated area of the tracks.
3. Metro also began addressing this recommendation when it began work with a contractor, ARINC, and other outside vendors to develop a real-time monitoring system that would detect circuit failures and generate alerts.
"The second NTSB recommendation was to ‘develop a program to periodically determine that electronic components in your train control system are performing within design tolerances.' Metro also is addressing this recommendation by working with Alstom Signaling, Inc., the manufacturer of the track signals that exhibited the failure, and with another signal manufacturer, US&S, to eliminate the problem identified.
"The NTSB investigation is not complete," Catoe said. "We will continue to cooperate with the NTSB and respond quickly in hope that they can identify a root cause or causes that will allow us to put steps in place to prevent this from happening again."