The crime happened on a train car at about 11:45 p.m. on Saturday, police said.
A 41-year-old man was approached by a group of unknown people when a "physical altercation" began, police said.
The victim was battered and was taken to the hospital with swelling to the forehead. He was expected to be OK.
Nobody is in custody. Chicago police are investigating.
INTERACTIVE SAFETY TRACKER Track crime and safety in your neighborhood