The shooting happened just before 8 a.m. on the Market-Frankford Line train at the 52nd Street station.
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It started as a fight between two men and quickly escalated when two more men got involved.
"Erupting into a full-blown three-on-one assault. One of those individuals produced a handgun, firing a round, striking one of the assailants in the arm," said Chuck Lawson, the acting SEPTA police chief.
It is not clear what led up to the shooting, but dramatic video showed a scuffle on board the train before you hear the shot.
Minutes later, you hear panicked passengers at the 52nd Street Station worried for their lives as another man stands holding a gun.
A group of people can be seen holding the suspected shooter down on the platform while officers run up to the scene. Police confirmed late Friday afternoon that the person shot was one of the people holding the shooter down until police arrived.
One of the officers recovered a gun and is seen unloading it. The suspect is then handcuffed and carried away. All three people believed to be involved in the initial fight were detained. Police are searching for a fourth person.
The 19-year-old victim was shot once in the arm and is expected to recover.
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Some riders say they want to see more officers in and around SEPTA.
"The police is not doing nothing. The mayor is not doing nothing. The (district attorney) is not doing nothing," one man said.
SEPTA's GM, Leslie Richards said they are trying to combat crime by hiring more officers. This is the second shooting this week after two people were shot on Wednesday at the Snyder Avenue station.
"I think it's very clear that the perception of crime is preventing people from using our system," said Richards. "We really are a collateral victim from the crime that is going on in the city right now."
The details surrounding the shooting remain unknown.
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