An outbound Blue Line train stopped at Harlem station in Forest Park was hit at 8 a.m. by an out-of-service train going the opposite direction on the same track.
Officials are looking into why the out-of-service train was on the track in the first place, said CTA spokeswoman Lambrini Lukidis.
Forest Park Mayor Anthony Calderone said the 33 people injured were taken to 10 hospitals, but none was seriously hurt.
Blue Line service was suspended between Forest Park and Kedzie Avenue, and service was later resumed to Forest Park. Harlem Station was still being bypassed, and shuttle busses were being provided.