Chopper 7 was over the scene, where Chicago police and fire were seen investigating.
Just after 10:30 a.m., the CTA initially said service was suspended between UIC-Halsted and Damen. Trains were running between Forest Park and UIC-Halsted and between O'Hare and Damen only.
The CTA said there was a bus shuttle between Damen and UIC-Halsted while service was suspended.
Trains have started running again with residual delays, the CTA said just before 10:50 a.m.
Authorities did not immediately provide further information about the investigation.
This is a developing story. Check back with ABC7 for updates.