Police say the victim was on the train in the first block of East Roosevelt Road around 11 p.m. when an armed man attempted to rob him and a fight ensued.
The gunman and the victim struggled over the firearm and the weapons discharged. Neither of the men were injured by gunfire.
Officers responded to the Roosevelt Red Line platform and found the victim and the offender fighting.
The offender was arrested and charges are pending, police say.
The incident remains under investigation by Area Three detectives.