Fairfax County officials have chosen four sites between Tysons Corner and Herndon in the Dulles Access Road corridor for express bus stations that could be converted to rail stops if funding becomes available.

Approval of a $36 million Urban Mass Transportation Administration grant, to be matched by county funds, is crucial to the Fairfax plan, which envisions full operation of the bus system by mid-1994 and conversion to a train line between the West Falls Church Metro station and Dulles International Airport at an unspecified date.

Most of Fairfax's share would come from bonds approved by voters in November. The county faces a Dec. 31 deadline to apply for the federal grant.

As endorsed this week by the Fairfax County Board of Supervisors, the county system would include a major bus transfer center at Spring Hill Road, an 850-space park-and-ride lot just west of Hunter Mill Road, a transfer center just south of the Reston Town Center mall, and a 1,790-space park-and-ride lot at Monroe Street, near Herndon.

In addition to expected federal funds and matching county money, part of the state's $10 million annual profit from the Dulles Toll Road will be available to finance bus and rail service in the Dulles corridor.

State officials are also developing a long-term proposal for transit in the 15-mile stretch, and Fairfax officials say their plans do not conflict with the state approach.

Meanwhile, the private Dartrail firm based in McLean has issued an alternative proposal for the heavily traveled corridor between the West Falls Church Metro station and Dulles International Airport that would be dependent upon many of the same funding sources but include money from a special taxing district of businesses along the Dulles Corridor.

Curtis Coward, an attorney for Dartrail, said he wanted state officials to have the company's ideas as the state prepares its own proposal for public comment in 1991.

The Dartrail plan also would start with buses and eventually would run elevated trains from the Metro station, through Tysons Corner, and then a ground level track along the Dulles Toll Road median from Route 7 to the airport. The Fairfax plan contemplates following the Dulles highway corridor from West Falls Church, skirting Tysons, to Dulles.

Planners consider smooth connection between the Metro system and the Dulles rail line critical to the success of the extension, regardless of who builds it. Both the Metropolitan Washington Airports Authority and Virginia Gov. L. Douglas Wilder have urged the rail line's construction. Wilder has pledged rail service to the proposed Dulles annex of the Smithsonian's Air and Space Museum.

Rep. Frank R. Wolf (R-Va.) has been prodding both the federal transit agency and Fairfax officials to pursue the bus system, which Fairfax officials estimate will cost $82 million to plan and execute and about $4.4 million annually to operate. Estimates to build a rail system in the Dulles corridor have ranged from $200 million to $700 million.

In making their bus station choices, the Fairfax supervisors turned down a citizen panel recommendation to put a park-and-ride lot near the Center for Innovative Technology, close to Route 28.