The incident was reported at the Times Square-42nd Street station on the Q train platform just before 6:30 p.m.
Police said the 43-year-old victim was approached by an unidentified suspect before he was slashed in the back.
They say the incident appears to be an unprovoked attack and that no words were exchanged between the suspect and victim.
He was taken to an area hospital and was said to be stable.
The suspect ran away from the scene.
Few other details were released.
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