IT COULDN'T BE said with authority even 18 months ago that Dulles International Airport was blossoming into a first-class hub of world travel -- but it's happening. The airlines are moving in and the passengers are moving in and around Dulles in increasing numbers. And for the first time in years of talk about a high- speed rail link to this airport, there is some high-level private interest in building a system between the West Falls Church Metro station and the airport.

A serious group of investors is proposing to do the job, suggesting that its proposed link could be up and running within five years if -- and this is the "if" to circle -- they can overcome strong local opposition and win enough federal backing. The proposed Dulles Access Rapid Transit would run along the existing median strip of the Dulles Access Road, at speeds of up to 75 miles an hour, both as an express between Metro and Dulles and as a local stopping at maybe five stations along the way. Estimated fare one way: $3. Estimated cost: $160 million to $200 million, proposed to be made up of shareholder equity, fees from developers with an interest in this corridor and payment from the companies that build and operate the system.

There's the considerable matter of raising capital, which DART hopes to do by selling bonds that would be backed by some combination of federal, state or local governments. There are some big hurdles: 1) the Reagan administration likes private enterprise in transportation, but no one in the administration is ready yet to like federal guarantees on loans for it (and even with federal participation, how many costly federal strings would be attached?); 2) there's some interest, too, on te part of Virginia Gov. Gerald Baliles, but here again, how eager is Richmond to put the state behind it? and 3) there's Fairfax County, which may be the biggest stumbling block of all.

County officials -- not the least of them Board of Supervisors Chairman John F. Herrity -- have many other transportation projects that they believe deserve more urgent financial and political attention, starting with a Springfield bypass and including other road projects and subway construction. DART may well work, and Mr. Herrity isn't saying don't. "If the Commonwealth of Virginia and the federal government want to guarantee debt service on bonds and guarantee operating costs, then Fairfax will cooperate."

A rail line to Dulles is a good idea so long as it is genuinely financed by private money and does not threaten to fall back on government money and thus jeopardize competing transportation projects in Northern Virginia.