The CTA Blue Line train that hit a westbound train near Harlem Avenue had been in service and started moving when it shouldn't have, according to CTA officials.
The four-car train collided with an eight-car train going west just before 8 a.m. CTA Spokesman Brian Steele said two switches that should have stopped the train from leaving the yard didn't, but that CTA staff is looking into every possible reason for the crash.
He said CTA does not know whether it was a runaway train, or whether anyone was on it at the time.
Ronald Ester, VP of CTA Rail Operations said the train, which had been in the Forest Park rail yard a few days, was going about 20 mph when it hit the westbound car.
Thirty-three people have been transported to 10 area hospitals with injuries following the crash. Steele said none of the injuries was life-threatening.