The stabbing happened on the train after two people got into an argument around 1:13 p.m.
The male suspect stabbed the 37-year-old victim multiple times on the train, police said.
After being injured, the victim ran off the train at the Wilson Red Line station.
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Police found him in the 4700-block of Broadway. He was taken to an area hospital in critical condition.
No one is in custody. Chicago police are investigating.
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