The CTA resumed service early Thursday after an equipment problem caused delays on the Red Line.
Fire officials told NBCChicago there was light smoke in the subway tunnel around 2 a.m. at the Roosevelt Red Line station. Several people were evacuated and crews took two people to Mercy Hospital and Medical Center for smoke inhalation.
Red Line trains were rerouted above ground to "L" tracks between Fullerton and Cermak-Chinatown. The CTA ran shuttle buses between 35th Street and Fullerton Avenue, as well as some shuttles on the Green and Orange lines, also experiencing some delays.
The Chicago Tribune reports the smoke was caused by a small fire from a very minor derailment.
Service was mostly back to normal ahead of the rush-hour commute, but CTA warned of a few delays.