Police received a call about 7:20 a.m. about someone who had been stabbed on the train. Officers responded to the Roosevelt station in the South Loop, 1167 S. State St., and found a 23-year-old man who had been stabbed in the neck and ear, police said.
A 26-year-old man was taken into custody, police said.
The 23-year-old victim was taken to Northwestern Memorial Hospital, where his condition stabilized, police said.
Area Central detectives are investigating.
The Sun-Times Media Wire contributed to this report.