It happened around 5 a.m. Friday on a train near the 46th Street Station at 46th and Market streets.
SEPTA officials say preliminary information indicates the victim was stabbed in the hand during an altercation on the train.
The victim was taken to the hospital for treatment.
SEPTA says a suspect was identified and taken into custody.
Westbound service on the Market-Frankford Line was operating with delays due to police activity.
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