An argument over a subway seat erupted in deadly violence early Saturday when a straphanger stabbed a stranger on a D train in Manhattan.

Cops quickly nabbed the 37-year-old suspect, thanks to a train conductor who locked the doors and kept him inside the blood-spattered car until police arrived.

The slaying happened near the 53rd St. station about 2 a.m.

The 36-year-old victim had a bag on a seat where the attacker wanted to sit. When he refused to move it, the man pulled out a knife and stabbed him several times in the neck and face, police said.

"The doors on the train were locked and the perp couldn't get away," a source said. "that's how we got him."

The man was pronounced dead on the northbound train, where pools of blood were left along the length of the first car.

The name of the victim and his accused killer have not been released.