A group of investors, including Washington lobbyist Robert K. Gray and former Organization of American States secretary general Alejandro Orfila, has proposed building a $300 million rail line linking the West Falls Church Metro station with Dulles International Airport.

The group has asked the Federal Aviation Administration to lease it the median strip on the Dulles Access Road where it proposes to build the 16-mile rail line, according to sources familiar with the plan. It is also seeking to lease from the FAA about 600 acres at the airport, which would be subleased to developers for office buildings, hotels and other projects to finance the rail line's construction, sources said.

The idea of a mass transit link to Dulles has been considered in the past by Metro planners and rejected because of its high cost. Private investors have also explored the possibility, but the Gray group has advanced further in its planning and has received encouragement from Reagan administration officials eager to involve private investors in mass transit projects, sources said.

If built, the Dulles rail link would be one of the nation's first privately financed rail lines since the 1930s, transit experts said.

The investors have formed a company called Dulles Access Rapid Transit, or DART, to develop their proposal. DART's directors, according to incorporation documents on file in Richmond, are Gray, chairman of Gray and Co., a leading Washington public relations firm, and a Republican insider; Orfila, one of Gray's partners, and Dulles-area developers Bahman Batmanghelidj and Joseph Walker.

Other partners, a source said, include Washington investment banker Carl Blake and three other businessmen. Blake, the company's spokesman, said DART is engaged in "delicate negotiations" with federal officials, and declined to comment on the plan.

Several local officials and business owners who have been briefed about the plan welcomed it as the best transit solution for the booming area of western Fairfax County. Others said the project would be an unnecessary expense for a relatively lightly populated region.

"It's illusion and dreaming," one Virginia developer said. "It's $200 million to $300 million you don't need to spend . . . . Dulles already has the best access of any airport anywhere."

But John F. Herrity, chairman of the Fairfax Board of Supervisors, said the plan "sounds like a very attractive proposal that would have to be seriously explored by Fairfax County . . . . It sounds pretty neat."

Sources said DART has proposed building a light rail line rather than the so-called "heavy" rail used by the Metro system. Light rail refers to a range of technologies, from trolley cars to monorails, but the exact technology that the investors propose using could not be learned.

The most recent proposal to build a Dulles rail line was advanced by Fairfax developer John T. Conlan. But federal and local officials said they did not take Conlan's proposal as seriously as they take the DART project because they believe his was not as well financed.

Aides to Rep. Frank Wolf (R-Va.) said that Conlan told them in December that he was dropping his plan to build the line. Conlan said last week that he still plans to push his project, but that it is "pretty much on hold."

DART representatives have told local Virginia officials that DART has the endorsement of Transportation Secretary Elizabeth Hanford Dole, FAA Administrator Donald Engen, Urban Mass Transit Administrator Ralph Stanley, Sen. John W. Warner (R-Va.), and Wolf.

An FAA source said, however, that Engen has not endorsed the plan. A DOT spokesman said Dole's position could not be learned immediately. Stanley could not be reached for comment yesterday.

Peter Loomis, a Warner spokesman, said that Warner supports the idea of private funding for a light rail to Dulles, and that while he has not given any "formal endorsement" to DART, he "certainly supports their efforts."

Jim Boyle, Wolf's spokesman, said that Wolf "is pleased there's private-sector interest" in building a Dulles rail line, but that Wolf would not endorse any particular Dulles rail plan until an UMTA study on the whole concept is finished. Sources said UMTA is expected to begin the study this spring.

Plans to build a rail link to Dulles have been floated for more than 20 years. FAA officials envisioned a rail line on the Dulles Access Road's median strip years ago, and dedicated it for that purpose. Metro officials at various points planned to extend the subway's Orange Line to the airport, but decided instead to extend it to Vienna. The Orange Line extension is expected to open in mid-1986, including the West Falls Church station, which is the proposed switching point between Metro and DART.

Transit experts said any rail line to Dulles would have to be aided by "feeder" bus lines that would take riders to rail stations along the way.

Fairfax Supervisor Martha Pennino said she thinks the idea of a privately financed railway is "absolutely fantastic . . . . The concept is right and the time has come. People of financial means are showing interest, and there's a great deal of interest by the government."

Pennino said proposed development near the 10,000-acre airport, including the Center for Innovative Technology and planned office complexes along Rte. 28, will necessitate mass transit.

Fairfax Supervisor Nancy Falck said county officials would be "very nervous" about such a proposal.

Falck said that riders would be discouraged by the inconvenience of a multistep trip into Washington that would involve taking a feeder bus, boarding DART, transferring to Metro, and then possibly taking a second train. Falck added that she is an advocate of buses on the Dulles corridor because their routes are flexible and they are cheaper to operate.

Other critics said that rail lines have an unproved track record at airports.

In light of the Reagan administration's opposition to financing urban transit, transportation specialists believe that mass transit systems will increasingly have to turn to private financing.

UMTA used to finance 80 percent of the capital costs of a number of urban rail lines, including Metro. But DOT officials say those days of mass transit subsidies are over, and they are looking for private money to back a rail project as an example of the Reagan administration's plan to "privatize" the country's infrastructure.

Some federal officials are backing a plan to finance a Dulles rail line by levying a $1 tax on each passenger at National or Dulles airports, sources said. Another alternative being examined by transportation experts involves partly financing a railway with a special real-estate tax on those who own land near the planned rail stations. The idea is that their land increases in value because of its proximity to the rail station.