On a typical day there are about 30 chances for a serious environmental accident at the Potomac Yard, the 500-acre railroad yard that lies a stone's throw from Crystal City, the Pentagon and many homes and schools in Alexandria and Arlington.

That is the number of cars carrying materials rated as "hazardous"--compressed gas, flammable liquids, corrosive chemicals and sometimes radioactive waste--that shuttle through the yard on an average day.

Each car carrying such a cargo bears a diamond-shaped placard warning of its contents. Yard crews say they handle the cars with special care, assigning them special positions in the trains assembled there for the long haul up or down the major rail line on the Eastern seaboard.

Since it was created in 1906, Potomac Yard has never recorded a significant accident involving hazardous materials, rail officials say. But with thousands of people working and living nearby, national safety experts say the potential for catastrophe is there.

This morning, the National Transportation Safety Board convenes two days of hearings into how to reduce yard accidents involving hazardous materials and how to improve emergency response to ones that do occur. Potomac Yard and other yards located in urban areas will be examined.

"There's a low probability of incidence, but a high probability if one occurs for considerable damage," says board member Patricia Goldman, who will preside at the hearings.

The hearings' target is accidents such as one that devastated a Houston rail yard in 1974. A collision punctured a tank car carrying the flammable substance butadiene, producing a mammoth fireball that killed one person, injured 235 and destroyed or damaged more than 500 cars.

Potomac Yard, largely hidden from view by trees and earthen mounds, is one of the largest facilities of its type on the East Coast, handling about 1,400 cars daily. Known as a "classification" yard, it breaks trains down into individual cars, which are formed up into new trains and sent on.

It is owned by the Richmond, Fredericksburg & Potomac Railroad and operated on contract for five other railroads.

Yard superintendent John F. McGinley says the yard has averaged two minor spills or accidental ventings of hazardous materials per year in recent years. In 1982, for instance, yard workers discovered small amounts of petroleum naphtha had leaked through bolt holes of an overloaded tank car.

A 1981 study by Virginia Tech scientists concluded that the yard was "most hazardous in Virginia" in connection with accidents involving the materials. From 1975 through 1979, it said, there were 166 reported accidents there, of which 20 involved hazardous materials in some way.

McGinley responds that the yard is by far the largest one in Virginia and understandably leads in the number of mishaps. But it has never had a significant accident involving hazardous materials, he says. "I stand on the record," he said.

Compared with many other U.S. yards, Potomac Yard receives a smaller portion of dangerous cargoes, about 2 percent of all cars.

McGinley argues that only about 20 percent of the 30 hazardous material cars Potomac handles daily contain high-test products.

About once every two months, according to McGinley, cars pass through the yard carrying radioactive waste.

Safety board analysts agree that Potomac Yard has a good safety record, but argue that it, like many others in the nation, has built-in features that worsen the potential for disaster.

Once located in open land, it is now hemmed in by office buildings and homes.

Alexandria Fire Chief Charles H. Rule estimates that 25,000 people would be affected if an accident forced the evacuation of everyone within a mile radius. "You've got Crystal City, you've got the Pentagon," says Rule, " . . . some pretty high-powered places."

Alexandria's city operational plan includes contingencies for evacuating residents from a hazardous material accident.

The yard also straddles two jurisdictions, Alexandria and Arlington. This raises the possibility, the board thinks, for confusion over who would respond to a call for help and who would be in charge at the scene. McGinley says he believes coordination is adequate.

Occasionally, fire officials stage surprise drills. In one 10 days ago, Alexandria fire officials say, rail yard employes on duty were unable to give the location of a car assumed to be involved in an accident.

Despite these concerns, safety at Potomac Yard has rarely been a subject of concern among citizen associations in adjoining neighborhoods.

Indeed, many people prefer the trains to development that would inevitably come were the yard closed. "It's not high and it doesn't generate traffic," said Alexandria Vice Mayor James Moran, who lives a quarter-mile from the yard. "It's preferable to expansion of Crystal City."

Recently, RF&P rearranged some tracks and freed up 50 acres of land to be added to Crystal City. Construction has begun there on a joint-venture project, which calls for four condominium buildings and five office buildings.

With the railroad also seeking rights to develop a 38-acre tract adjoining the George Washington Memorial Parkway, speculation has arisen that the RF&P might eventually close down the rail operations and open up the land for full commercial development.

But RF&P president John Newbauer says the yard is going to stay put. "I view Potomac Yard as a continuing and very important interchange facility for the next 18 or 20 years, probably even beyond that," he said.