The Chicago Fire Department said the smoke was caused by a trash fire along the tracks near the North/Clybourn Red Line station. Firefighters opened a vault in the 1200-block of Clybourn to clear the subway of smoke.
The fire department said all smoke has been cleared from the subway as of 12:15 p.m. No passengers had to be evacuated from trains and no injuries were reported.
Red Line service resumed its normal route via subway shortly after the smoke was cleared.