A man was killed by a subway train in Brooklyn yesterday after he jumped down on the tracks — possibly to retrieve his dropped keys.

The victim, who was not carrying identification, was struck at around 5:30 a.m. by a Canarsie-bound L train at the Halsey Street station in Bushwick, cops said.

MTA employees told The Post that the man, who law-enforcement sources said had been intoxicated, had been trying to get his keys after they fell onto the tracks. He died at the scene.