The incident happened just before 3 p.m. on the 2200 block of North Broad Street underground on the Broad Street Line.
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According to SEPTA, the victim, a 23-year-old man, got into an argument on the street.
The argument continued into the North Philadelphia Station, where the victim was shot once in the right chest.
He was transported to Temple University Hospital by 22nd district officers, where he was placed in critical condition.
At the time of the shooting, authorities confirmed other people were on the platform.
So far, no arrests have been made.
Officials believe part of the incident was caught on tape.
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Some people who ride the subway believe situations get worse during the warmer months, so they try to keep to themselves.
SEPTA says they are working to hire more police officers and security personnel, but some believe it's not enough.
"I don't feel safe in the city at all, period. It's just too much stuff going on," said Makiyh Butler of West Philadelphia.
Anyone with information is asked to contact police.