It wasn't clear when results of the tests would be available, but the board scheduled a late afternoon news conference Wednesday.
The crash Monday occurred just after a train operator, who had been on the job about three months, started her shift. About eight miles down the red line track, another train was stopped. In the blink of an eye, the trailing train, slammed into a stopped one and jackknifed violently into the air, falling atop the first. Nine people were killed, including the train operator, and more than 70 were injured.
"It was only a split second," said 20-year-old Jamie Jiao of Vienna, Va., who was aboard the moving train, sitting just a few feet from where the car was smashed. "We were probably traveling pretty fast. No one had time to react."
Investigators are trying to find out why the train didn't stop, even though officials say the emergency brake control was pushed down and there was evidence that the operator tried to slow her car before impact.
"That train was never supposed to get closer than 1,200 feet, period," said Jackie Jeter, president of a union that represents Metro workers.
Emergency brake controls are referred to as "mushrooms" on transit trains, protruding from the operator's console so they can be slammed down with a swift slap of the palm.
Hersman said Tuesday it wasn't clear when the button was pressed or how it got that way. Much on the train, including toggle switches and other controls, could have been disturbed in the crash, she said.
Safety officials also are investigating maintenance records and a passenger's statement that the train had stopped briefly then started again before the accident.
The horror of the worst disaster in the system's 33-year history shocked the Washington region, where the Metro system is an integral part of the economic and social fabric.
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.