RailCorp has been ordered to pay $650,000 to a man who was hit by a train after drunkenly falling on the tracks at Mortdale station, with a court finding the driver breached his duty of care by failing to use the emergency brake as soon as he saw an object on the rail line.

Shane John King sued RailCorp for $1.3 million, but in the NSW Supreme Court on Friday Justice David Davies awarded him half that amount to account for his "contributory negligence" in being drunk.

Mr King lost part of his left leg when he was struck and run over by the train in Sydney's south in the early hours of September 2, 2006.

He told the court he drank three longnecks of beer while visiting a friend that night, and can't remember the accident.

CCTV footage at the station showed Mr King appearing to kneel at the edge of the platform, before almost "somersaulting" onto the tracks and landing with his left leg over the top of a rail, the court heard.