Virginia officials today threw their support behind a $4 billion, 24-mile rail project that would connect Metro to Tysons Corner and Dulles International Airport, despite warnings from the Bush administration that the transit proposal is one of the most expensive in the country and too costly to win federal approval.

A top official at the Federal Transit Administration, speaking on condition of anonymity, said this week that the projected ridership did not justify the plan's price tag and that there was no way the agency could recommend it for federal funding. "The costs overwhelm the fairly significant benefits," the official said.

But the unanimous vote by the Commonwealth Transportation Board, a 17-member panel appointed by the governor to oversee highway and transit projects, keeps alive a rail project conceived in the 1950s and now heralded by Virginia as the best way to loosen the traffic congestion that threatens to paralyze the state's high-technology corridor and one of its main economic engines.

"This means that rail is not dead and people can hope to see it sometime in their lifetime," said Karen Rae, director of Virginia's Department of Rail and Public Transportation. "To do nothing would mean congestion in that corridor would grow and grow."

At $4 billion, the project would be more expensive than the reconstruction of the Springfield interchange and the Woodrow Wilson Bridge combined.

The rail extension is costly because of its length -- it would represent about a 25 percent expansion of the Metro system -- and the number of rail bridges and tunnels required to weave it through densely developed Tysons Corner. Metro has projected ridership at 71,900 daily trips, but transit officials said new data will boost those numbers.

The state's decision to move ahead with Dulles rail comes a month after Northern Virginia voters refused to tax themselves to help pay for that and other transportation projects. An economic slump has evaporated tax revenue and forced Gov. Mark R. Warner (D) to slash state spending.

Critics of the project say state officials wrongly endorsed a Cadillac solution when a Ford would do. They want a newer, more flexible system known as bus rapid transit that they say could serve the Dulles corridor at one-tenth the cost.

"The political momentum has built up over the years for rail," said John H. "Jack" Rust Jr., a Fairfax lawyer and former state delegate. "The federal government was trying to give them a reality check. I think they missed it."

Bus rapid transit, relatively new to the United States, uses specially designed buses that operate in their own lanes and behave like trains on rubber tires.

The backlash against rail has forged some unlikely alliances. Bill Vincent, a former Clinton administration transportation official who now works for a nonprofit agency that promotes bus rapid transit, is joined in opposition by Bruce Fein, a former Reagan official who now is a conservative legal commentator.

The most difficult stage in the project's progress lies ahead. Virginia must find a way to make the rail plan more palatable to the federal government so it can get permission to begin preliminary engineering. Approval from the Bush administration is key because it would pay as much as half the $4 billion cost while state and local governments and the airport pick up the rest.

The Federal Transit Administration has advised the state to segment the project, suggesting that Virginia might be able to proceed if it builds the railroad in phases. That's the way the current Metro system was built -- one chunk at a time, over the course of 25 years -- because the federal government refused to fund the $10 billion, 103-mile system all at once. Many transit projects across the country, including subways in Los Angeles and Atlanta, have been built the same way.

One option, for instance, would extend the Orange Line to Tysons Corner and, after that work is completed, submit the rail segment from Tysons to Dulles for federal approval.

But chopping up the Dulles project could rip apart its delicate web of financial and political support. The players behind the rail project -- state agencies, county governments, members of Congress, airport officials and the commercial landowners along the proposed route -- hold distinct interests and have agreed only on building rail to Dulles.

Fairfax County has been planning to raise its share -- $514 million -- from new taxes on commercial property in the Dulles corridor. But commercial landowners in the Reston-Herndon area are unlikely to agree to tax increases to pay for a rail line that would run only to Tysons Corner with a vague promise that it would reach them one day.

"If it becomes two or more projects, rail to Tysons and a promise to take rail someday out to Loudoun County, I don't think that can be sold," said A. Linwood Holton Jr., a former governor and Metropolitan Washington Airports Authority chairman who teamed up with former U.S. senator Charles S. Robb to lead a group of commercial landowners in the Dulles corridor.

Similarly, the airports authority, which controls the Dulles Access Road, has said it will not allow a rail line to be built there if the line runs only to Tysons.

And state officials have said that their strategy to generate Virginia's $800 million share of the project from higher tolls on the Dulles Toll Road may be in jeopardy because raising fees during an economic downturn is politically difficult.

"The big $64,000 question is how can you phase something over time that ultimately moves people in the corridor, that promotes sound land use and environmental benefits, and it's something people want?" said one local transit official, speaking on condition of anonymity.

Sources close to Warner said the governor is likely to get involved in project negotiations that he had largely left to staff. He is well acquainted with the rail project, having worked on a Dulles rail task force a decade ago as a business leader in Northern Virginia. He is determined to make some improvement in the corridor, sources said.