The shooting took place around 9:45 a.m. Sunday at the 52nd Street Station.
According to police, two males appeared to have a confrontation on a westbound train.
As one of the men exited the train, the other fired his gun several times, striking the victim in the back.
Officers released surveillance images of the suspected shooter. He can be seen wearing a tie-dye sweatshirt and dark-colored beanie.
Police say they found the victim on the street outside the station and rushed him to the hospital.
He is in critical but stable condition.
Riders told Action News they're always looking out for their safety.
"More recently now, I've been more cautious in the past month," noted Hmo Tu from University City.
"I'm more worried when it gets too late, just for safety. I'm coming back from work, I don't want anything to happen to me or my bag or belongings," added Leslie Okeke from University City.
Anyone with information is asked to call SEPTA police at 215-580-8111.