An official report on the number of casualties was not immediately reported. News media reports ranged from 16 to more than 40. Those injured were reported in good to fair condition, WBBM-AM, Chicago, reported.
WLS-AM, Chicago, reported multiple ambulances were called and the injured were taken to area hospitals.
A westbound train was stopped at the Harlem station in Forest Park when it was struck by an eastbound train, the Chicago Sun-Times reported.
A CTA spokeswoman said it wasn't known why the two trains were on the same track.
Service was temporarily suspended between Forest Park and Austin, transit officials said.
Taylor Pettiford, 17, who was waiting at the station, told the Chicago Tribune she heard screaming as she waited for the train for her commute to Muchin College Prep.
"Stop! Stop! Stop the train!" she said she heard people yelling. "Slow down!"
Pettiford said she heard a crash and more screaming.
"I instantly started crying and I called my mom to come pick me up," Pettiford said.