It happened along the Red Line in the 1600-block of West Howard Street.
A man in his 40s was standing on the platform when he slipped, fell and landed on the tracks.
CTA had to shut down power on the Red Line while paramedics tended to the person, which caused some delays during the morning commute.
The man, who was reportedly unconscious, was taken to Saint Francis Hospital in Evanston. His current condition is unknown.