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On Monday, the National Transportation Safety Board (NTSB) issued an "urgent" safety recommendation to the Washington Metropolitan Area Transit Authority (WMATA) calling for enhanced train-control system safety redundancy in the wake of a major Red Line accident that occurred on June 22.

Although the board's accident investigation isn't complete and no probable cause has been determined, investigators have concerns about WMATA's train-control system safety redundancy, the NTSB said.

Board officials believe WMATA's system is susceptible to a single-point failure because it did not fail safe and stop a train after losing detection of a preceding train. The NTSB is urging WMATA to evaluate track occupancy data on a real-time basis to detect losses in track occupancy and automatically generate alerts to prompt actions, such as immediately stopping trains or implementing appropriate speed restrictions to prevent collisions.

The NTSB also issued a second urgent recommendation to the Federal Transit Administration (FTA) calling for the agency to advise all transit-rail operators with train-control systems capable of monitoring train movements to evaluate the systems for adequate safety redundancy.

"By calling upon WMATA to take swift action to upgrade the safety redundancy of its system and by urging the FTA to alert other transit agencies of the hazards of single-point failures, such as the one experienced by WMATA, we hope to prevent something similar from happening again," said NTSB Acting Chairman Mark Rosenker in a prepared statement.

WMATA officials "appreciate" the recommendations issued by the NTSB, however the train-control system issue "has implications that reach far beyond Metro, and extend to the entire transit industry," according to a statement released by the transit agency.

"This is not a simple problem as is evidenced by the NTSB's statement that 'investigators are continuing to examine the train-control system circuitry and recorded data to better understand how the train-control system functioned prior to the accident,'" WMATA officials said. "It is important to know that there are currently no systems available commercially that could provide the Metro system with the kind of alerts that the NTSB has recommended, and that such a system must be invented."

WMATA plans to develop a train-control system specifically tailored to Metro and is contacting vendors "who have the expertise needed to help us develop this service," they said. The agency also plans to establish an independent panel of experts in signal systems through the American Public Transportation Association before the NTSB completes its investigation.