Stessel declined to comment on initial reports of what happened. He said the train is at the New Carrollton rail yard, undergoing “further inspection.”
“We are looking at the switch and the track area where this occurred,” he said. “We’re talking to the train operator and any other employees involved. Once that data is collected, we’ll be able to draw some conclusions as to the cause, but it is still early at this point.”
The train was carrying about 1,000 people at the time, and no injuries were reported, Stessel said.
Metro officials could have more details on the cause in days, he said.
Matt Bassett, chairman of the Tri-State Oversight Committee, which monitors safety at Metro, said the investigation would narrow the cause.
“There is still work to do on how human performance, work processes and equipment may or may not have contributed” to the derailment, he said.
Bassett said the committee sent officials to the scene Tuesday night to assist Metro.
Multiple sources close to the investigation, who spoke on the condition of anonymity, offered similar descriptions of what happened based on initial reports.
Shortly after 7 p.m., an operator pulled a six-car train into Rosslyn station, a key transfer point for the Blue and Orange lines. Passengers disembarked. As the train operator was preparing to pull away from the station, she noticed a red signal on the switch. Switches send trains in the proper direction and are generally considered among the sections of track most vulnerable to derailment, sources said.
The operator called Metro’s operations control center to report the red signal. The sources said the operator was apparently told by the control center that there was a switch problem. The train operator was authorized to get out of the train and obtain help from a nearby crew to manually put a clamp in place to hold the switch in the correct position. The action was supposed to allow the train to advance on the Blue Line toward the Franconia-Springfield station.
It is unknown why the switch was not working, the sources said.
Usually, a computer controls switch movements, according to rail experts. If a switch is not working or a clamp is improperly put in place or fails to work, that can cause a derailment, they said.
In Tuesday night’s incident, once the train operator and crew finished, the control center gave the operator permission to slowly proceed out of the station with the train. She did, but within seconds, the front wheels on the first rail car derailed, the sources said.