The project to extend Metrorail through Tysons Corner would be exempt from new federal cost standards under the transportation bill Congress passed last week, effectively removing a key guideline it would have flunked.

The exemption, tucked into the 321-page legislation, allows the controversial project to win federal funding despite scoring only a "medium low" rating for cost-effectiveness. A recent change in federal standards called for projects to receive a cost-effectiveness rating of "medium" or better.

"It means we don't have to meet the new criteria," said Marcia McAllister, a spokeswoman for the Dulles Corridor Metrorail Project, an arm of the state government. "The same rules for cost-effectiveness that we were subject to last year continue to apply."

Sen. John W. Warner (R-Va.) inserted the provision. "This is not an exemption to cost-effectiveness standards," said his spokesman, John Ullyot. "Senator Warner wanted to make sure that the rules were not changed on us in the home stretch."

Three other rail projects, in Silicon Valley, San Francisco and Oregon, won the same exemption.

Critics of the project, now estimated to cost as much as $2.4 billion, said the exemption allows an unworthy project to win federal money.

As one of the most expensive proposed rail lines in the country, the Tysons project has been the focus of an almost philosophical battle over the competing virtues of road and rail projects.

"It's outrageous," said William Vincent, who supports a bus rapid transit connection over rail. "Those cost-effectiveness standards are in place to protect the taxpayers from waste and abuse. The exemption is an admission that this project is wasteful."

Cost issues have dogged Northern Virginia's rail effort, which eventually is supposed to extend Metro's Orange Line from about West Falls Church through Tysons Corner to Dulles International Airport and Loudoun County.

The hefty price of that 23-mile line has forced organizers to break the project into two phases, the first stopping roughly halfway, at Wiehle Avenue on the eastern edge of Reston.

But now even the first phase is running into cost troubles.

In June, the engineering firms developing the plan told Virginia transportation leaders that the project, as envisioned, probably would cost $2.4 billion, a 60 percent increase from the previous estimate.

Since then, engineers have been working to make significant cost cuts, for two reasons.

First, the $2.4 billion price is well beyond the federal, state and local funds lined up for the rail line; and second, the project had to meet federal cost-effectiveness standards to win the federal share.

The exemption essentially lowers the cost-effectiveness standards by which it will be measured.

In April, the administrator of the Federal Transit Administration, Jennifer L. Dorn, issued a letter saying it will target funding recommendations to transit projects that win a "medium" or higher rating for cost-effectiveness.

The new legislation releases the Tysons rail project from that restriction.

Dan Scandling, spokesman for Rep. Frank R. Wolf (R-Va.), a supporter of the project, defended the exemption as keeping the rules consistent, rather than having them change in "midstream."

Leaders of the project said they must cut the project's cost, regardless of the exemption.

Under the financing plan for the Tysons portion, previously 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 route, who have agreed to pay a special real estate tax.

"We still have to separate the wants from the needs," McAllister said. "It needs to be affordable."