The two men were in the midst of a dispute when the attacker slashed his 23-year-old victim across the left leg on the northbound train at the West 125th Street station at about 2 a.m., sources said.
The victim was taken to St. Luke’s Hospital and is in stable condition.
Cops arrested the suspect at W. 127th St. and St. Nicholas Ave. shortly after the slashing.
The suspect had a knife, police said.
It’s unclear what sparked the argument.