RICHMOND, SEPT. 20 -- Virginia officials pledged the profits today from the Dulles Toll Road toward construction and operation of a rail line linking Metro and Dulles International Airport, marking a major philosophical shift from the state's traditional emphasis on highways to solve traffic problems.

No detailed rail proposal has been developed, though state officials hope to present one for public comment by early 1991. Officials envision expanding express bus service and adding three new park-and-ride lots as an interim measure by the mid-1990s. The lots could be converted to rail stations as the line is built.

Today's unanimous vote by the Commonwealth Transportation Board does not guarantee that the 15-mile Tysons Corner-to-Dulles rail line will be built. Additional funds will be needed from local governments, and possibly from a special tax district in which owners of commercial land who stand to benefit agree to help finance the project.

The toll road surplus is now about $10 million a year and is expected to exceed $20 million a year within six years. The state action designates 15 percent of the profits for mass transit use immediately, with the percentage gradually rising to 85 percent as rail station sites are identified and an operator is chosen. Virginia officials plan to begin work on the transit system in July 1991.

Estimates of the cost of extending rail service to the airport have ranged from $200 million to $700 million. Meanwhile, a study suggested that an express bus system that would handle nearly as many riders could cost $72 million. Fairfax officials have been working on a bus system, but have not identified specific park-and-ride sites. The county plans a $36 million November bond referendum and may apply for a $36 million federal grant for bus and rail transit in the corridor.

"It's an extraordinarily good idea," said Fairfax County Board Chairman Audrey Moore of the decision to use the toll road surplus. She said the money could back loans that would get the project underway.

The Metropolitan Washington Airports Authority, which owns Dulles and National, has demanded mass transit money from the state in exchange for permitting the profit-making Toll Road Corp. of Virginia to extend the current toll road through Dulles property to Leesburg.

Linwood Holton, the authority's board chairman, noted that today's transportation board resolution is not a binding contract, but may be strong enough for the authority to allow the toll road extension to proceed if remaining technical issues are resolved with the Toll Road Corp.

The toll road extension has been strongly supported by Gov. L. Douglas Wilder and by State Transportation Secretary John G. Milliken, a former Arlington County Board member and Metro Board chairman. They also hope to make good on a pledge to the Smithsonian Institution that rail service would be provided to the planned Air & Space Museum annex at Dulles.

Early this year, Virginia officials said all profits from the current toll road would be needed for improvements to the highway and its interchanges for many years. But after he took office in March, Milliken signaled a new approach to confronting gridlock in what is expected to be one of the region's most heavily traveled transportation corridors.

Milliken said he believes that the trains can start rolling by the end of this decade.

For decades, Northern Virginia legislators battled for state government contributions to the Metro system, and finally won them. In recent years, many local officials have emphasized their belief that new highways alone cannot solve the region's traffic problems. Their focus has turned increasingly to car-pool lanes, bus service and new rail lines, such as the planned rail service that would link Fredericksburg and Manassas with the District.

State officials say it is not likely that the Dulles rail service would be run by Metro, but acknowledged that it would have to connect smoothly with the West Falls Church Metro station to be successful.

The median of the parallel Dulles Airport access and toll roads has been preserved for a rail line.

The plan is advancing at a time when many long-planned transportation projects are being delayed across the state.

The toll road surplus is not part of the state's general highway fund, is not subject to planned statewide spending cuts and by law can be used only in the Dulles corridor.

The current toll road is being widened from two lanes to three in each direction, with the shoulders to be available as fourth traffic lanes at rush hour.

Several interchanges are being improved, automated toll collection equipment is planned and rush-hour car-pool restrictions are scheduled to take effect next year.