The incident was reported on a downtown train at 59th Street and Lexington Avenue just before 1:30 p.m.
A man was slashed with a knife and suffered a laceration to the head.
He was taken to the hospital and was said to be stable.
Cops are looking for three suspects in connection to the incident -- two men and a woman.
The motive in the attack remains unknown, but police believe it was completely unprovoked.
No arrests have been made.
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