It happened near the Harlem station at approximately 11:30 p.m. Monday.
The CTA had tweeted that service was down to a single track in some places, but a CTA spokesperson now says service has returned to normal. Some minor residual delays were reported.
While the repair work was under way, shuttle bus service was running between the Rosemont and Jefferson Park stops.
The incident came on the coldest morning in two years -- with temperatures below zero degrees in some places overnight -- and certainly the wind chills far colder than that.
The bitter cold also was causing other problems Tuesday. In southwest suburban Joliet, a water main broke.
The dangerous wind chills were making going outside a challenge for anyone. Those who have to brave the cold are urged to bundle up and protect their skin.