Metro officials yesterday endorsed one of the priciest transit projects in the country -- rail to Tysons Corner and Dulles International Airport costing as much as $4 billion -- while acknowledging they can't afford their plan to maintain the existing Metro system, handle future crowds and expand transit service over the next decade.

"There's no question about it, we're going to face a crisis in funding for transit," said Metro board Chairman Chris Zimmerman of Arlington, describing how local governments have only $2 billion of the $12.2 billion needed for new rail cars to run eight-car trains, improvements to stations to handle crowds and proposed light rail and bus projects as well as rail to Dulles.

Still, they unanimously approved the capital plan as a guide and agreed to proceed with part of it. "It's clear this is a sound plan and a vision for where we need to proceed," said John Davey, who represents Prince George's County. "It's equally clear none of the jurisdictions have the resources."

The dismal financial forecast didn't deter the Metro board from unanimously accepting the most ambitious way to bring mass transit to the Dulles corridor: a 24-mile rail extension from West Falls Church to Route 772 in Loudoun County, with four of its 11 stops in Tysons Corner.

"It's a little like saying, 'If we had ham, we could make ham and eggs, if we had eggs,' " Zimmerman said. "But we have to move ahead."

Karen Rae, the director of the Virginia Department of Rail and Public Transportation, said yesterday's decision will allow engineers to draw specific designs and will force officials to come up with detailed financial plans.

"All this does is move it to the point where answers can be found to all these critical questions," she said.

Metro and Virginia's government studied several options, including a bus rapid transit system between West Falls Church and Loudoun that would cost about $481 million. Metro's choice of rail is one of two key endorsements needed before the project can be considered by the federal government for approval and funding. The other must come from the Commonwealth Transportation Board, a group of gubernatorial appointees that will consider the project next month.

J. Kenneth Klinge, a member of the transportation board and a key backer of the rail project, predicted the region will meet the financial challenges, which include creating a tax district along the corridor, raising money from tolls and locking in nearly $1.5 billion from the federal government.

Critics said the Metro directors endorsed a budget-busting project at a time of shrinking revenue across the region.

"It's disappointing to see this mixed message," said Bill Vincent of the Breakthrough Technologies Institute, a Washington-based nonprofit group that promotes new environmental and energy technologies. "On one hand, we barely have enough money to maintain the existing system, and on the other hand, we're going to shoot for this pie-in-the-sky $4 billion investment."

Metro managers said the project would cost $3.3 billion, assuming the rail line opens in 2010 and the federal government pays half the cost, or $200 million a year.

But a top federal transit official, speaking on condition of anonymity, called that scenario "really unlikely" and said a more realistic level would be $100 million a year. If that occurs, the state could borrow money against eventual federal payments to keep construction moving but would incur $700 million in financing costs, driving the total cost to nearly $4 billion, said John Dittmeier, Metro's acting project manager for Dulles.

While Metro officials are clinging to a 2010 completion, state and federal officials have suggested that 2016 is a more realistic target.

Vincent said Metro failed to take a serious look at another way to transport people that is cheaper, faster to implement and growing in popularity across the country: bus rapid transit, or BRT. BRT uses specially designed 60-foot buses that run frequently in reserved lanes and stop at stations. In some places, BRT can carry as many passengers as a rail system at a fraction of the cost.

Metro officials never considered running bus rapid transit through Tysons, saying it wouldn't work because of the traffic around the shopping centers and office parks. Katherine K. Hanley (D), the Fairfax County Board of Supervisors chairman who represents Fairfax on the Metro board, said landowners in the Tysons area don't want a bus system and refused to help pay for it.

Metro directors, who represent the governments in the agency's service area, agreed to go ahead with part of their capital plan: Metro will maintain the existing system while directing its staff to come up with a strategy to find money for the rest of it, including the purchase of 252 rail cars so the subway can operate eight-car trains.

The plan, designed to position the transit system a few steps ahead of the growth curve by 2013 instead of lagging behind with crowded rail cars and buses, is divided into three parts.

The first calls for $3.3 billion to maintain the system by replacing and rehabilitating aging buses, rail cars, elevators, escalators, mechanical systems, maintenance facilities, tracks, stations and parking garages. Local governments have promised to pay about two-thirds of this first phase.

The second, $2.9 billion part calls for the purchase of 252 rail cars and 460 buses, the expansion of rail yards and bus garages and improvements to cramped stations such as new entrances, extended platforms and underground walkways between Gallery Place and Metro Center and between Farragut West and Farragut North.

The last part is $6 billion for new service. Projects include Dulles rail, trolleys on Columbia Pike in Arlington, trolleys between the Anacostia waterfront and Minnesota Avenue in the District, and the Purple Line light rail between Bethesda and New Carrollton.

Staff writer Michael D. Shear contributed to this report.