A Long Island fire official who had been scamming $3,000 a month in disability benefits through the LIRR “gravy train” scheme is hoping he has a prescription for avoiding prison — by fingering the doctor who allegedly signed off on his bogus ailments.

Robert Ellensohn, 59, pleaded guilty in federal court yesterday to conspiracy and fraud as part of a deal to testify at the upcoming trial of Dr. Peter Lesniewski, who feds say helped hundreds of LIRR workers rip off the federal Railroad Retirement Board.

Ellensohn, a former LIRR inspector, admitted “filling out paperwork falsely” and said he “submitted false documents filled out by others” when he took early retirement in 2004.

Ellensohn — a former North Merrick Fire Department chief — said he’s been collecting a $5,600 a month pension, more than half from crooked disability benefits.

His crimes carry a maximum 45 years in the slammer, although prosecutors plan to seek leniency in exchange for his cooperation.

Under terms of his deal, Ellensohn also agreed to forfeit more than $300,000 he swindled.