CONCORD, CALIF., SEPT. 1 -- A Vietnam veteran kneeling on a railroad track in an effort to stop a munitions train outside the U.S. Naval Weapons Station was critically injured today when he was run over by the train as onlookers and family members watched in horror.

S. Brian Willson, 47, of San Rafael, suffered a skull fracture and lost one leg below the knee at the scene. His other leg was amputated below the knee during eight hours of surgery, according to a spokeswoman at John Muir Hospital.

Willson, leading about 25 demonstrators protesting alleged arms shipments from the Concord station to Central America, was kneeling on the track after a news conference when the engine hit him.

He and another protester were to begin a 40-day "Fast for Life and Peace" at the base about 40 miles east of San Francisco. Willson engaged in a lengthy fast last year in Washington.

Contra Costa County Sheriff's Capt. Gary Ford said the incident was "just a tragic accident."

He said the investigation may be hampered because the train's crew refused to be interviewed or tested for drugs or alcohol until Navy counselors arrived.

Dan Tikalsky, a spokesman for the weapons center, said the crew members later were interviewed and that drug tests may be ordered.

"We are cooperating fully with the investigation," he said.

Tikalsky said the train operator, a civilian employe, applied the brakes when he realized not all the protesters would be off the tracks at a road intersection on county property adjacent to the weapons center.

The train was traveling about 5 mph, he said, adding that he did not know when the brakes were applied on the straight stretch of railway, where visibility was several hundred feet.

Base commander Lonnie Cagle said officials were told that some protesters jumped onto the tracks before the train struck Willson.

Other witnesses, however, said Willson and seven others were on the tracks when the approaching train sounded its horn. Three demonstrators immediately jumped off the tracks and four others fled as the train neared. Willson rose from his seated position, but was hit before he could jump off, they said.