The 17-year-old was on the train near the Union Street station in Park Slope when it happened shortly after 9 p.m.
Police say the teen asked three men to turn down the music they were playing before an argument ensued.
That's when the victim was stabbed in the leg.
He was taken to a nearby hospital, where he is reportedly stable.
Police say three male suspects fled the scene, and officers are still trying to track them down.
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