A migrant from Guatemala has been arrested on allegations he lit a sleeping woman on fire inside a Coney Island F train on Sunday morning.
Police sources say a civilian identified the suspected killer as he rode on a train at 34th Street in Manhattan.
The witness waved down the cops, who later picked the suspect up and detained him.
He has not been charged with a crime yet, sources added.
He is suspected of lighting the woman on fire at about 7:30 a.m. on Sunday, then watching her burn to death in a shocking killing just three days before Christmas.
Cops briefly talked to him, but told him to clear the area in the immediate aftermath of the attack, a video from the scene showed.