A teen was busted Thursday for allegedly setting a sleeping homeless man on fire on board a train passing through Midtown this week, law enforcement sources said.

Cops nabbed the 18-year-old accused maniac in the East Village and were expected to charge him for the harrowing Monday attack, sources said.

The unhinged firebug stepped into a No. 3 train approaching 34th Street-Penn Station just after 3 a.m. and set the snoozing 56-year-old victim ablaze before taking off, cops said.

The attack left the victim so badly scorched that cops found skin on one of the seats on the train, sources said.

He was taken to NewYork-Presbyterian/Weill Cornell Medical Center with significant burn injuries, cops said.

Surveillance photos and video released by police a day later showed the suspected arson

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