Blue Line trains were standing at Western Avenue just after 3 p.m., the CTA said.
Chicago fire officials said the person who was on the tracks was taken to Stroger Hospital in critical condition.
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It was not immediately clear how the person got on the tracks along the Eisenhower Expressway.
Just before 4 p.m., the CTA said trains were moving again.
This is a developing story; check back with ABC7 Chicago for updates.