A federal safety expert said yesterday that Alexandria and the Potomac rail yard do not have adequate plans for responding to potentially catastrophic accidents involving explosive, poisonous or other hazardous shipments at the 500-acre yard.

Addressing a hearing convened by the National Transportation Safety Board, board investigator Quinten Johnson said Alexandria has no "comprehensive emergency plan" for Potomac Yard, which handles about 30 carloads of hazardous materials daily.

Other testimony brought out that rail yard officials have not discussed evacuation plans with adjacent National Airport, a pipeline carrying jet fuel runs through the yard and inspectors have occasionally found unmarked shipments of hazardous materials.

Alexandria Mayor Charles E. Beatley told the hearing at the Crystal City Marriott that the city's fire department does have special procedures for the rail facility, which has never had a significant accident involving hazardous materials. But he described them as "fairly general."

After the hearing, Beatley said the city would tighten coordination and seek more information on dealing with hazardous shipments from the yard's operator, the Richmond, Fredericksburg & Potomac Railroad. "We'll take the initiative," he said.

During the hearing, called by the board to focus attention on safety issues at Potomac Yard and others like it in the U.S., industry officials argued that the country's railroads have maintained an excellent safety record, moving about a million carloads of hazardous materials annually.

But Johnson said a survey of eight selected U.S. yards showed a potential for catastrophe and a need for better planning. "The level of overall preparedness we found in these locations is not encouraging," he said.

Alexandria's plan, he said later, fails to spell out in sufficient detail such concerns as who will be contacted in an emergency, what buildings will be evacuated and what streets used, and which yard officials firefighters would deal with. "That does not instill a high degree of confidence," he said.

Yard superintendent John F. McGinley said at the hearing that his facility had worked closely with Alexandria and Arlington on emergency plans, providing tank cars for training exercises. Recently, he said, the yard spent $156,000 laying a new water main for use in firefighting.

John Spink, assistant chief of the Arlington Fire Department, said in an interview that his department has a "quite extensive" 49-page emergency plan applying just to rail facilities in the county. He said that adding further detail to it probably would not be practical.

In the event of an emergency, Spink said, the Arlington Fire Department would be responsible for ordering and coordinating any evacuation of National Airport.