The engineering firms developing the plan to extend Metrorail through Tysons Corner told Virginia transportation leaders yesterday that the project as envisioned will probably cost $2.4 billion, a 60 percent increase over the previous estimate and a price that far outstrips the carefully negotiated financing agreement.

Project leaders said they would immediately start work on revising the scope of the construction to reduce that figure, but it was not immediately clear how much cost-cutting would be feasible.

"There's no way we can build a project that costs that much money," Sam Carnaggio, the state's project director of the Dulles Corridor Metrorail Project, said of the $2.4 billion figure. "Now's the time to separate the needs from the wants."

The escalating estimate creates an array of political and practical complications for government and civic leaders in Northern Virginia, who have called the project a top priority for the traffic-choked region.

The project eventually would extend 23 miles from West Falls Church through Dulles International Airport, but planners have split it into two phases, the first through Tysons, because of concerns about the cost.

The engineering firms reported yesterday that the cost of the Tysons rail portion could be held at about $1.7 billion, an increase of roughly 15 percent, but only by altering the project in several significant ways, including replacing the proposed mile-long tunnel through Tysons with above-ground tracks, eliminating two pedestrian bridges to the stations on Route 123, reducing train platform sizes and deviating from Metrorail construction standards.

It is too early to tell whether the changes, outlined by the engineering group known as Dulles Transit Partners, would be acceptable to Fairfax County, the state and Metro, all of which have a stake in the outcome. Officials said they were reviewing the proposals.

The first phase of the line would extend from West Falls Church through Tysons to Wiehle Avenue, in the Reston area. Opponents have criticized the segment as far too pricey, given that it is likely to attract about 15,100 new daily riders. Metro averages nearly 660,000 passengers daily.

Yesterday's report is likely to be seized upon by those who have been calling for the rail money to be spent instead for roads, innovative bus systems or other transportation solutions.

"This should come as a wake-up call," said William Vincent, an advocate for a system known as Bus Rapid Transit, which has buses running in dedicated lanes and stopping at stations in a manner similar to trains. "It demonstrates the need once again to come up with a more cost-effective strategy."

Under the financing plan for the Tysons portion, which previously had been estimated to cost $1.5 billion, about half the money would come from the federal government, about one-quarter from Dulles Toll Road collections and other state revenue, and about one-quarter from commercial property owners along the train route, who have agreed to pay a special real estate tax.

The new cost estimates were made as engineers develop more detailed drawings for the project, in a phase known as preliminary engineering.

If the price rises significantly, the project's state and local advocates will be forced to go back to those sources and ask for more money, or find other sources.

"The state is going to have to go back and make those numbers work," said Dan Scandling, a spokesman for Rep. Frank R. Wolf (R-Va.), who has been instrumental in pushing for federal financial support for the project. "This is the state's project. They hired the engineers and put the financing plan together. They have to explain how these numbers are going to work -- or are not going to work."

The escalating price also affects a critical cost-effectiveness measure that the Federal Transit Administration uses in weighing whether to help pay for it.

In March, the FTA announced that it would target its funding recommendations at projects that garner rating of "medium" or better for cost effectiveness.

Even when the cost estimate for the Tysons rail project was $1.5 billion, the FTA ranked its cost effectiveness as medium-low. The $2.4 billion figure would drop that assessment down another notch.

Acknowledging that a significantly higher price would fail federal standards, Carnaggio said: "I think we can get to a lower figure. But it's going to take a lot of work over the next two months."

One of the key players in the coming months will be Metro, which will be asked to allow the new rail line -- an extension of the Metrorail system -- to deviate from its standards in order to save construction money.

Richard A. White, Metro's chief executive, said yesterday that his staff would review the cost-cutting proposals.

"We're obviously going to evaluate every idea on that list," he said. "There are a lot of value judgments that need to be made."

Fairfax County Board of Supervisors Chairman Gerald E. Connolly (D) similarly said he was open to changes, even the idea of eliminating the tunnel through Tysons Corner. Engineers advocated construction of the tunnel, saying that placing the tracks above ground over a hill in the area would mean elevating them 50 feet or more in places -- a potentially unattractive structure in what boosters hope will become Northern Virginia's "downtown."

"We can live with that," Connolly said of eliminating the tunnel. "Getting this project done is essential to our future."

"There are a lot of value judgments that need to be made," said Richard A. White, Metro's chief executive, referring to possible cuts in the project.