Rail traffic through the Potomac Yard, the Eastern Seaboard's gateway to north-south freight traffic, slowed to a trickle yesterday and some area employers said they will be forced to lay off thousands of workers or shut down operations if the rail strike continues another day.

Hardest hit were Virginia port cities and Baltimore, as well as Virginia's coal mining region, already staggered by a long coal strike and the 79-day strike at Norfolk and Western, the nation's largest hauler of bituminous coal.

Most passenger traffic in and out of Washington remained snarled, with no rail service to the south, and only one train going west - to Chicago. Service to points north of Washington remained unaffected by the strike.

Commuters crowded buses between Washington and Baltimore and area Greyhound stations reported a surge in travelers to the south. Truck companies said their phones were ringing off the hook as callers tried to switch cargo from rail to truck.

A skeleton crew of 11 supervisors yesterday manned the critical Potomac Yard switching station in Alexandria, trying to take the place of 110 to 115 workers who refused to cross a Southern Railroad employes' picket line.

The yard superintendent set up bunks and hired a catering service for the supervisors.

"There are times when things get a little hareied," said howard Beach, assistant yard superintendent. "I wouldn't say it's going as smooth as silk."

Only eight trains passed through the yard yesterday, according to trainmaster Maury Barksdale, compared to the usual 50 to 80 headed north and south.

With delays caused by other unionstruck operations, little of the grain, paper, steel, appliances and gas that move by rail was getting throught to the industries and ports that depend on them.

A spokesman for Baltimore's General Motors assembly plant, which manufactures auto engines, axles, frames and other parts, said his company would lay off 6,200 of it 7,000 workers tomorrow if the strike continues. The plant might close down entirely by the weekend, he said.

A spokesman at one of Baltimore Harbor's three grain elevators said his company had lost nearly $30 million in business because of the 2-month-old Norfolk and Western strike Tuesday had "virtually cut off all business.Right now nothing is moving," he said.

The spokesman, who asked that his company not be named, said he had canceled a contract not be named, said he had canceled a contract for 15 ships to carry grain, and laid off an entire 50-man shift. Further layoffs seemed imminent, he said, if the strike continued. Other grain dealers in the port said they were similarly affected.

Shippers dependent on goods moved in bulk such as coal, ore, oil and grain were the first to feel the pinch.

Hampton Roads, Va., already losing about $500,000 a day in revenue from curtailed coal shipments stands to be among the most vulnerable of Virginia's areeas, according to Norm McTeague, operations officer for the Virginia Office of Emergency and Energy Services.

Norfolk has suffered dramatically from the long rail strike. In June, 44 ships loaded with coal left the port, officials said. In August, only three coal-bearing ships left the harbor. The broadened strike threatens to snuff out what little coal traffic remains, according to port spokesmen.

Statewide, more than half of Virginia's 16,000 coal miners and operators, already shaken by a long coal strike, are out of work because coal strike, are out of work because coal already minded cannot be shipped.At a maximum of $110 a week per worker, they are collecting more than $1 million a week in unemployment compensation from the state.

In Roanoke, headquarters of Norfolk and Western, the Roanoke Valley's largest employer, department stores and restaurants report a sharp decline in business. The city's revenue commissioner said he is expecting a large volume of tax delinquencies when real estate taxes come due in December, and many families have gone welfare.

"We are a very proud population, but they are being forced to take food stamps against their will," said the city's vice mayor, Charles Landis. "We're just holding our breath and hoping for the best."