Commuter rail service for Northern Virginians, once a mere twinkle in a transportation planner's eye, moved a step closer to reality yesterday.

At its meeting in Richmond, the state transportation board voted to assume responsibility for the design and construction of seven commuter parking lots to serve a proposed 90-mile system, which would provide service to downtown Washington on two corridors, from Manassas and Fredericksburg.

"That is really a tremendous boost to us," said Stephen Roberts, who is managing the project for the Northern Virginia Transportation Commission.

Commuter rail service still faces a range of obstacles. Funding is not assured, nor has the question of liability insurance for the railroads that own the track been entirely resolved.

Nonetheless, state officials have grown increasingly committed to the service out of concern over mounting traffic congestion on both the I-66 and I-95 corridors. Last month, the transportation board voted to provide a $3.2 million grant to the transportation commission so it could begin purchasing equipment and building railroad platforms.

"The metropolitan region is expanding and even though we're making road improvements, there will still be terrible congestion," said Roberts. "It's been demonstrated clearly that the road network has to have this kind of auxiliary."

In other action yesterday, the board awarded 111 highway contracts around the state worth $99 million. It was the largest number of contracts awarded in a single month in the history of the Virginia Department of Transportation, officials said, and reflects the impact of the $422 million-a-year transportation tax package approved in a special session of the General Assembly last year.

In Northern Virginia, the state awarded a $6.9 million contract for the widening of Rte. 28 for two miles north and south of the Dulles Access Road and construction of a mile-long, four-lane access roadway from Rte. 28 to the planned Center for Innovative Technology.

The widening project, which will add two lanes to Rte. 28, is part of a massive series of improvements planned for the corridor that will cost in excess of $300 million.

Yesterday's vote on the rail service commits the state to assume responsibility for managing construction of the parking lots, a step that will speed the project once the necessary funding is secured, according to Roberts.

The commission estimates that it can provide the service for $9 million a year, paying for it with $4 million in passenger fares and the rest in state and local funds.

The commission hopes to begin service in March 1989 using eight trains, four on each line.

Under the proposal, the commission would buy cars and locomotives and lease time on tracks owned by Norfolk Southern, Conrail and the Richmond, Fredericksburg & Potomac Railroad. Amtrak would operate the service under contract.

A ridership estimate released last month predicts that the service would carry 4,000 people a day, which exceeds the traffic-carrying capacity of a new freeway lane during the busiest rush hour, according to the Northern Virginia Transportation Commission.

In a prepared statement, Transportation Commissioner Ray D. Pethtel noted that the I-95 corridor already carries about 100,000 vehicles a day, a number that is expected to reach 150,000 by 2000.

Roberts said the commission and the railroads are nearing agreement on the question of liability insurance, although he added, "any of the parties could still mess it up."

The project also must overcome the reluctance of officials in Fredericksburg who fear their picturesque community will be overrun each morning by commuters driving there to catch the train, he said.