Northbound Red Line subway trains between Fullerton and Cermack were rerouted to the elevated tracks for about 40 minutes.
CTA said a bus shuttle service will be available between the Fullerton and Clark/Lake stops.
At around 2:50 p.m., CTA tweeted that the Red Line was returning to normal service, but it would take a little while to get fully back to normal.
Chicago fire officials and CTA officials did not immediately confirm what caused the loss of power.
No injuries were reported.