The transit agency tried to assure riders their trains were still safe and all were running on manual control Tuesday as a precaution against computer malfunction.

When the train is in automatic mode, the operator's main job is to open and close the doors and respond to emergencies.

The crash occurred near the D.C. and Maryland border, in an area where higher train speeds are common because there is a longer distance between stops. Trains can go 55 to 59 mph, though it was not clear how fast the train that crashed was traveling.

The cars in the moving train were some of the oldest in the transit network, dating to the founding of the system in 1976.

Federal officials had sought to phase out the aging fleet because of safety concerns, but the transit system kept the old trains running, saying it lacked money for new cars.

Hersman told The Associated Press that the NTSB had warned in 2006 that the old fleet should be replaced or retrofitted to make it better able to survive a crash.

Neither was done, she said, which the NTSB considered "unacceptable."

Metro General Manager John Catoe said the agency expected to receive proposals "over the next month or so" to replace the old cars, but new trains were still years away from being installed. He insisted the existing cars were safe.

This isn't the first time that Metro's automated system has been questioned. There was a close call in 2005 because of signal troubles in a tunnel under the Potomac River. Signal relays that control trains were replaced after a serious safety warning in May 2000 by the Federal Railroad Administration. The only other fatal crash was in 1982, when three people died in a derailment.

The operator of the train that barreled into the stopped cars was Jeanice McMillan, 42, of Springfield, Va., who was hired in January 2007 as a bus driver. She was tapped to become a train operator in January 2009, the NTSB said. McMillan completed training and began working as an operator in March.

Metro spokeswoman Candace Smith said there has been no indication that McMillan was sending text messages or talking on a cell phone, which contributed to passenger train crashes in other cities, though federal investigators were checking her phone records.

Metro allows operators to carry phones but they are not allowed to use them while working. The NTSB says it routinely requests cell phone records after crashes.

Associated Press writers Brian Witte, Brett Zongker, Matthew Barakat, Gillian Gaynair, Alex Dominguez and Sagar Meghani contributed to this report.