A commuter train is likely to have been unmanned when it crashed into another train outside Chicago during Monday's morning rush hour, officials said.
Chicago Transit Authority (CTA) spokeswoman Lambrini Lukidis said video from the scene shows no driver at the controls of an empty train that crashed into another train carrying passengers.
As many as four dozen people were hurt, although officials said none of the injuries were life threatening.
Ms Lukidis said the video has been turned over to the National Transportation Safety Board, which is investigating.
The empty train was eastbound when it hit a westbound train that had stopped at the Blue Line's Harlem station in Forest Park. The CTA said the station handles around 4,000 passengers each weekday.
Forest Park is about 10 miles west of central Chicago.