Scores of others were injured in the 5 p.m. EDT collision of two six-car Red Line trains between the Fort Totten and Takoma Park stations near the D.C.-Maryland line, The Washington Post reported.
The trains were both headed in the same direction and collided with such force that the front end of one ended atop the rear of the other, officials said. One of the fatalities was the operator of the trailing train, the newspaper said.
D.C. Fire Chief Dennis L. Rubin said an estimated 70 people were injured -- six of them critically. Emergency crews cut apart the trains to get victims out.
The cause of the accident had not been determined. Metro spokeswoman Lisa Farbstein said something "obviously … went terribly wrong for two trains to be on the same track."
Metro General Manager John Catoe said the leading train had stopped because a third train was stopped at the next platform and the trailing train "for reasons we do not know, plowed into that train."
Washington Mayor Adrian Fenty told reporters it was the deadliest crash in the history of the D.C. Metrorail system.
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