A U.S. judge yesterday set aside a jury verdict that had awarded $608,000 in damages to a 14 year-old boy whose left leg was severed as he tried to hop a freight train here five years ago.

The Baltimore and Ohio Railroad Co. had been ordered by the jury last July to pay the money to Myron Alston, after Alston's attorney argued that the railroad company should have placed signs or other warning devices around the track involved since it knew children played there.

In setting aside the verdict, U.S. District Judge Oliver Gasch said the jury's findings extended beyond the proper scope of the law concerning the liability of a firm for such an accident.

"It has never been part of our law that a landowner may be liable to one who, after unlawfully entering his land, proceeds to wantonly expose himself to unmistakable danger in total disregard of a fully understood risk, simply for the thrill of the venture," Gasch ruled.

Gasch recounted testimony from the trial in which Alston and his friends acknowledged going to the tracks in northeast Washington on numerous occasions to hop freight trains.

The accident occurred on June 27, 1972, in the vicinity of 9th and Kearney Streets NE.