Chopper 7 was over the scene at North Franklin and West Illinois streets in River North just before 8 a.m.
Video showed a man on the tracks near a stopped train, as emergency crews gathered around.
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A police tactical team used a ladder at one point to reach the man, who was eventually taken into custody.
CPD said the incident involving a 22-year-old man began just after 7 a.m.
He was taken to Northwestern Memorial Hospital for observation, police said.
No one was injured in the incident.
Brown and Purple Line trains were moving again about 8:20 a.m. after the incident, Chicago police said.
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