A man slashed another straphanger in the head aboard a Manhattan subway late Wednesday — and then escaped onto the tracks, authorities said. 

The suspect, described as in his 40s, got into a squabble with the 21-year-old victim on a southbound A train as it moved through the tunnel, close to Penn Station, around 9:30 p.m., cops said. 

As the dispute escalated, the assailant pulled out a knife and slashed the victim in the forehead, cops said. 

At some point during the fracas, someone pulled the emergency brake, bringing the train to a halt, police said. 

That gave the suspect the chance to bolt, apparently through an open door somewhere on the train, and onto the tracks, authorities said. 

He was still on the loose early Thursday. 

The victim was taken to Bellevue with a cut on his forehead, cops said.