The accident happened at 7:47 a.m. during rush hour when an eastbound Chicago Transit Authority train hit a westbound train that was stopped at the Harlem station in Forest Park, about 10 miles west of downtown Chicago. Thirty-three people were taken to hospitals, but their injuries are believed to be minor.
"I was waiting for my train. Time to go to school. I heard screaming, 'Stop the train, stop the train, slow down,'" Taylor Pettigrew, a witness, told Chicago affiliate WLS.
An official said the eastbound train was empty but traveling on the same track when it hit the westbound train.
The cause of the accident has not been determined. The Chicago Transit Authority is investigating the accident.
The Associated Press contributed to this report.