Commuter trains collide in Washington, D.C.

A rush-hour collision between two Metro trains on Washington, D.C.'s Red Line killed at least six and injured dozens Monday. (Pablo Martinez Monsivais / Associated Press)

Rescuers use ladders to rescue survivors of the Red Line crash from one of the trains. (Pablo Martinez Monsivais / Associated Press)

Rescuers use ladders to reach one of the Red Line trains. Witnesses said one train rear-ended another near the Ft. Totten station. (Win McNamee / Getty Images)

Rescuers carry one of the dozens injured in the crash. The operator of one of the trains was reported to be among those killed. (Jacquelyn Martin / Associated Press)

A body is carried from one of the Red Line train cars. (Alex Wong / Associated Press)

Rescue workers respond to the Red Line rail collision. It was the worst accident in the Metro train system’s 33-year history. The only other fatal accident was in 1982. (Win McNamee / Getty Images)

Cars from both trains were ripped open in the collision, and rescuers had to cut some people out. (Win McNamee / Getty Images)

Rescuers carry an injured man from the wreckage. Officials said they had no explanation for the accident yet. (Alex Wong / Getty Images)