A stranger slashed a 21-year-old man in the face in an unprovoked attack on board a Bronx train late Thursday, cops said.
The victim was riding a southbound No. 6 train passing through the Zerega Avenue station around 11:45 p.m. when a maniac randomly cut him on the left side of the face with a sharp object, police said.
The attacker got off the train, leaving the wounded man on board, authorities said…