A man is in critical condition after being struck by a 6 train at Lexington Avenue-59th Street on Saturday, authorities said.

The victim was on the tracks when the Brooklyn-bound train hit him at around 4:30 p.m., FDNY sources said.

He was transported to New York-Presbyterian/Weill Cornell Medical Center with life-threatening injuries.

Police are investigating the cause of the accident.

Due to the investigation, 6 trains are running express from 125 Street to Grand Central-42nd Street in both directions.