On September 30, 2013, two Blue Line trains collided at (Congress branch), injuring 33 people, after an outbound train was hit by an out-of-service train going the opposite direction on the same track. Earlier, the train had arrived at Forest Park, and went out of service. However, after the train pulled into the Desplaines Yard, it was left on. There were no passengers on the out-of-service train; the number of workers on it at the time of the crash was also an unknown. Also unknown was the total number of passengers on the outbound, in-service train. The investigation started shortly thereafter. Though service soon went back to normal, trains did not stop at the Harlem station until the late evening on Tuesday, October 1, 2013, to avoid interference with the scene and congestion. The NTSB concluded that the probable cause was water in the control cables and insufficient securing of unattended equipment.