CPD said the battery was reported just after 5:10 a.m. in the 1200-block of North Clark Street.
The victim was riding the Red Line when an unknown male suspect came up and hit him with an object that was possibly a hammer, Chicago police said.
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The man suffered a laceration to the forehead and was taken to Advocate Illinois Masonic Medical Center, where he was stabilized.
Area Three detectives are speaking to a person of interest, police said.
Howard-bound Red Line trains had some delays after the incident.