The stabbing happened at about 11:40 a.m. at the CTA Red Line 47th Station, police said.
A 48-year-old man was sitting in a train when three suspects approached him.
Police said they robbed him and one of the suspects pulled out a "sharp object."
After a fight, police said the victim was stabbed in the back.
He was taken to the University of Chicago Hospital; police said he was in good condition.
Chicago police said nobody is in custody.
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