The shooting happened on a train at the Cermak-Chinatown Red Line Station at about 3:48 p.m., officials said.
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ABC7 spoke with passengers who were on the train. They said two men got into an altercation, and one pulled out a gun and shot the other.
Police said 21-year-old man was shot in the calf. The Chicago Fire Department transported him to the University of Chicago Medical Center in critical condition.
The alleged shooter was taken into custody, and a weapon was found on the scene, police said. Charges are pending and all passengers were evacuated from the train.
The CTA said Red Line trains are running with delays at the Cermak-Chinatown station due to police activity, and crews working to restore service.
Trains, in both directions, are running on the same track between Roosevelt, Cermak-Chinatown and Sox-35th stations. The CTA asker riders to board and exit all trains on the 95th-bound side of the platform at Roosevelt, Cermak-Chinatown and Sox-35th stations.
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The 54th/Cermak-bound Pink Line trains are also running with delays.
Officials did not immediately provide further information about the shooting.
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