No one was injured and normal service was restored as of 4:40 p.m.
Firefighters were called to the tracks on the 900 block of West Diversey Parkway at 4:14 p.m., said Chicago Fire Department spokesman Meg Ahlheim.
A smoldering rail tie caused the power to be turned off in both directions at 4:25 p.m., affecting the Belmont, Wellington, and Diversey stations, said CTA spokeswoman Catherine Hosinski.
The fire involved rubbish and was extinguished at 4:37 p.m., said Ahlheim, who added that no one was hurt.
Power was restored and normal service resumed at 4:40 p.m., according to Hosinski.
Riders on the Brown, Red and Purple Lines experienced delays.