A group of private investors who plan to build a high-speed passenger rail line linking the West Falls Church Metro station with Dulles International Airport said yesterday the proposed system could begin operation within five years if the group overcomes strong local opposition.

The Dulles Access Rapid Transit Corporation (DART), led by Najeeb E. Halaby, former administrator of the Federal Aviation Administration, released details of its plan to run the rail along the median strip of the Dulles Access Road. DART said it would bring essential transportation relief to a corridor of Fairfax County that is growing too fast for other solutions.

But Fairfax County officials, who have questioned the ability of any private company to operate a profitable rail system without public assistance, said it was not a project local taxpayers wanted to pay for.

"We have our priorities and that is not one of them," said John F. Herrity, chairman of the Board of Supervisors. "As far as I can see, the [Springfield] bypass is the number one necessity in this century. That is what we want and that is what we are willing to pay for."

"He is acutely ill from Metrophobia," said Halaby in discussing Herrity's constant skepticism toward the rail system. "But this is not another Metro."

Fairfax County has poured millions of dollars into the Metro, far more than was originally planned. Herrity said that the county suffered a $37 million operating deficit on Metrobus and Metrorail in 1985 alone.

At a meeting with reporters and editors of The Washington Post, Halaby and two former secretaries of transportation, Alan S. Boyd and William T. Coleman Jr., began a public relations offensive by announcing that former Virginia governor Charles S. Robb and William P. Clark, a former national security adviser and secretary of the interior, had joined the group's board of directors.

Halaby said DART trains would run at speeds up to 75 miles an hour, stopping at five stations between the two terminals. The cost of a ticket from one end of the system to the other would be about $3, and 80 percent of the passengers would be commuters.

The idea of a mass transit link to Dulles is not new; it is in the Fairfax County Master Plan and was originally envisioned as a stop on the Metro system.

But the plan for private ownership has drawn steady support from Reagan administration officials who are seeking to encourage the development of transportation systems that would require no federal funding.

Last year, a Department of Transportation task force expressed its enthusiastic support for the concept of a privately funded high-speed rail line to Dulles. Ralph L. Stanley, head of the Urban Mass Transportation Administration, frequently has said that in the future, federal funds will be scarce for publicly operated mass transportation systems.

DART, which estimated the cost of the rail system at $160 million to $200 million, hopes to raise funds by a mixture of shareholder equity, fees from developers who stand to benefit from the construction, and payment from the companies that build and operate the system.

It also hopes to raise capital through selling bonds, which would have to be backed by the federal, state or local governments -- or some combination of the three methods.

Although Reagan administration officials would like to see a successful private rail line as a model of private enterprise in the nation's capital, it is the residents of Fairfax County who will be most difficult to sell on the idea.

"We don't envision increased development along that corridor," said Herrity, "and we are not going to pay for something the Commonwealth of Virginia wants. If they want it that badly I say I am for it. Just tell them to bring their checkbooks with them when they start construction."