The incident was reported at a station at Avenue of the Americas and Spring Street around 4 p.m.
A 58-year-old man on a northbound C train was approached from behind by a 20-year-old man and stabbed in the left leg, in what appeared to be an unprovoked attack.
The victim's injuries were not believed to be life-threatening, and he was taken conscious and alert to Lenox Hill Hospital.
The attacker got off the train at the Spring Street station and fled the scene. Authorities are looking for a man wearing black Adidias pants with a white stripe.
Subway changes and delays were reported after the incident but normal service resumed.
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