On January 7, 2007, about 3:45 p.m. eastern standard time, northbound Washington Metropolitan Area Transit Authority Metrorail train 504 derailed one car as the train traversed a crossover from track 2 to track 1. The accident occurred in an underground tunnel on the Metrorail Green Line near the Mt. Vernon Square 7th Street-Convention Center (Mt. Vernon Square) station at chain marker E2 23+28. The train was traveling about 18 mph as it approached the station.
The train consisted of six cars. The fifth car from the head end of the train derailed. About 80 passengers were on board at the time of the accident. Twenty-three passengers were transported to local hospitals for treatment and released. Emergency response personnel from Washington, D.C., provided the on-scene treatment and transportation of the injured passengers.