A CTA spokesperson said the extreme temperatures led to a cracked rail near the Kimball station.
The CTA temporarily suspended between Kimball and Southport, with trains are only operating between Southport and the Loop.
Shuttle buses were available between Kimball and Southport.
Shortly after 8:30 a.m. service was resuming with residual delays.
The CTA said it is working to restore service but it is not known how long the delay will continue.