Authorities said a suspect stabbed a 46-year-old man in the back around 11:30 p.m. Monday.
The victim was taken to the hospital in serious condition.
The attacker fled the scene on the 5200 block of Frankford Avenue and is still at large.
The stabbing is the latest act of violence involving SEPTA.
A week ago, a SEPTA employee was attacked at the City Hall station.
On Sunday night, a man was a shot at the 15th and Market stop.
SEPTA has shut down the Somerset Station in Kensington over safety concerns.
On Tuesday, riders impacted by the closure of the Somerset Station will voice their concerns.
A planned protest is set for 5:30 p.m. at the station along Kensington Avenue.
The station was closed to deal with pressing issues, including the need to clean up and make repairs because of homelessness and drug use in the area.