The crash occurred after 4 p.m. west of the VA-7 interchange.
Trains on Metro's Red Line were single-tracking between the Takoma and Silver Spring stations during the Friday evening commute.
The accident occurred before 8 a.m. near Boundary Channel Drive.
Shuttle buses are being brought in to replace trains.
Service was suspended on the two lines between the Archives and U Street stations because of an unauthorized person on the tracks.
Metro trains were single-tracking in part of downtown Washington during the Monday evening commute, bringing delays for riders of three lines.
Metro said trains on its Red Line were sharing a track between the Medical Center and Twinbrook stations Wednesday morning because of a disabled train.
Metro's Silver Line trains are operating only between the Wiehle-Reston East and Ballston stations.