Commuters were forced to get off the train at McPherson Square Metro stop during severe service interruptions after a train derailed Thursday morning. (Photo by Astrid Riecken For The Washington Post)

Metro crews were able to remove the final four cars of a train that derailed inside a tunnel between Federal Triangle and Smithsonian stations late Thursday night, but they still do not know what caused the train to leave the track, officials said Friday.

Metro spokesman Dan Stessel said the four cars were removed around 10 p.m. Those cars, along with two that were removed earlier on Thursday, have been sent to the rail yard at New Carrollton, where they will be inspected for damage and for clues as to what might have caused them to derail.

[Derailment forces commuters to scramble for alternatives]

Stessel said that some minor track repairs were done around 5 a.m. and that after a final track inspection around 6 a.m., the area was cleared to begin regular service.

No passengers were aboard the train and no injuries were reported, but the early morning incident left tens of thousands of commuters scrambling for a Plan B after service was suspended on portions of the Blue, Orange and Silver lines for much of the day. Even though trains were able to single-track through the area by the time the evening commute rolled around, many riders reported long waits, crowded cars and generally miserable conditions.

[After Thursday’s derailment another bumpy commute for Metro riders]

Officials were able to reopen the Smithsonian and Federal Triangle stations late Thursday. But even as crews were scrambling to restore full service on portions of those rail lines Friday morning, a power failure forced the transit agency to halt service between Ballston and West Falls Church on the Orange Line and between Ballston and McLean on the Silver Line. Passengers at East Falls Church were evacuated from a disabled train to the station platform. Service problems also were reported on several other lines.