The mid-afternoon shooting left riders unsettled and caused delays along the whole line due to the investigation on South Side.
"Yes, I'm shocked. But we must continue. I'm still going to use public transportation," said CTA rider John Chambers.
Police say the victim was shot in the chest and shoulder as he argued with another rider.
The altercation, police say, began at the station's turnstiles, continued onto the platform and then onto the arriving train where police say the shots were fired.
"Everybody started running for cover. Everyone was scared, started ducking and trying go up under the seats," said Cheri McKee, a CTA rider.
Police say the wounded man was taken to University of Chicago Hospital where his condition was stabilized.
The Garfield Red Line stop was closed for an hour-and-a-half as investigators did their work, forcing trains to bypass the station.
"I'm going to have to completely go another different route. I might be very late for work. I have to pick up my daughter. This is a big inconvenience," said CTA rider Rosie Ricks.
The gunman fled the scene, and is still being sought by police. Investigators were reviewing CTA surveillance video.
The Garfield station is located between the northbound and southbound lanes of the Dan Ryan expressway at Garfield Boulevard.