Police said it happened at the Belmont Station, which serves the Red, Brown and Purple lines, on Friday shortly before 4 p.m.
The victim was approched by a black man in his 20s, who “struck the victim multiple times to the face with closed fists,” police said.
The offender was wearing a black hat, black jacket, red pants and red shoes, police said.
The victim, who is in stable condition, was being treated at Illinois Masonic Hospital.
No arrests have been made, and Area North detectives are investigating.