The CTA said on X, formerly known as Twitter, that the incident began sometime before 11:40 a.m., and trains were standing at Roosevelt in the South Loop.
Chicago fire officials said a Green Line train "failed" near 13th Street and Wabash Avenue, and some riders were taken off the train due to heat.
Shuttle buses were available between Clark/Lake and Halsted for Orange Line service and between Indiana and Clark/Lake for Green Line service, the CTA said.
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The No. 3 King Drive and No. 62 Archer buses also might be alternate routes for riders, the CTA said.