The National Transportation Safety Board will hold two days of hearings later this month on safety procedures for handling hazardous materials at the Potomac railroad yard south of Crystal City and at other major rail yards around the country that are near populated areas.

Board spokesman Ira Furman said yesterday that he knows of no specific safety hazard or significant accident involving hazardous cargoes at the 500-acre Potomac Yard, which is owned by the Richmond, Fredericksburg & Potomac Railroad. But all yards that handle the materials have a potential for danger, he said.

"While rail transportation of hazardous materials is generally safe and recent years have seen improvements in tank car safety, the possibility of an accident turning into a disaster still remains," board member Patricia Goldman said in a statement.

The board's interest in the yard was prompted by its location near heavily developed areas and the fact that it lies in two jurisdictions, Arlington and Alexandria, which board experts believe might complicate response from local rescue teams in an emergency.

The Potomac Yard's location near the board's Washington headquarters also figured in the decision to examine it, Furman noted.

The board is currently investigating an April 3 accident in Denver in which more than 14,000 gallons of nitric acid spilled from a tank car, causing 37 injuries and leading to the evacuation of 9,000 people. That accident and others helped prompt a major inquiry into rail transportation of hazardous substances, Furman said.

Hazardous materials include flammable, toxic, corrosive and radioactive substances.

Potomac Yard superintendent John F. McGinley estimated that 10 to 15 of the approximately 3,200 cars that pass through the yard daily carry substances officially classified as hazardous. He said the yard fully meets federal safety standards and has cooperated closely with neighboring jurisdictions for emergency help.

Car valves occasionally have vented small amounts of hazardous gases by accident, he said, but the yard has never had a major spill or major accident involving the substances.

Furman said the hearings, which will be held on July 26 and 27 at the Crystal City Marriott, will examine issues such as tank car design, rail yard operations and safety procedures, and federal regulations.