Federal officials gave permission yesterday for Virginia to begin engineering work on the $1.5 billion plan to extend Metro to Tysons Corner and Reston, breathing new life into a massive public works project that appeared ready to collapse several times in the past year.

"We've always believed this corridor needs a public transportation solution," Federal Transit Administrator Jenna L. Dorn said. "They have a long way to go, as every project does when it begins preliminary engineering. But we feel confident they're on the right track."

The route would serve motorists, commuters and shoppers in one of the region's most congested corridors.

The approval means that the rail project, which has been discussed since the 1960s, will move beyond computer models for the first time. "We've got the beginning of a project -- not studies, not planning but engineering work," said Karen J. Rae, director of the Virginia Department of Rail and Public Transportation. "It's a pretty significant step forward."

Preliminary engineering is the second of three phases that a transit project must complete before it can be considered for federal construction money. Engineering, scheduled to take 15 months, is supposed to produce more accurate cost estimates as well as specifics on benefits and impacts.

Approval to start preliminary engineering does not guarantee that the federal government will decide to fund the rail project. But it is a crucial step in the lengthy process to win federal money.

"It's critical in that it separates this rail project from other competing projects that are not as far along, yet there are more hurdles that have to be overcome," said Richard A. White, chief executive of Metro, which has agreed to provide technical expertise on the project.

The rail project is a slimmed-down version of a $4 billion plan to extend Metro to Dulles International Airport, which Federal Transit Administration officials said was too expensive. Virginia officials devised a plan to build the line in phases, with a first section from West Falls Church to Wiehle Avenue. The second would run from Wiehle Avenue to Dulles International Airport and end at Route 772 in Loudoun County.

Federal officials made it clear they are backing rail to Wiehle Avenue only.

But Rae stressed that the state remains committed to building the entire line to Dulles and that Virginia will pay for preliminary engineering for the second phase.

"Dulles Airport is the largest airport in the nation's capital, and it's going to grow exponentially in next decade," said Fairfax County Board of Supervisors Chairman Gerald E. Connolly (D). "For reasons of national security, practicality, accessibility, you've got to serve Dulles with rail."

The state wants a private firm, rather than Metro, to design and build the project. It would be the first time a segment of Metro was not built by the public transit agency. Virginia officials said they have an innovative plan that will save tax money and set an example for other transit projects.

Opponents say it is a sole-source deal that would benefit Bechtel Corp. and Washington Group International, the companies that entered into a partnership to build the rail line.

State officials have been holding confidential negotiations with the private partners since January 2003 and have a draft agreement. There is no opportunity for public review until the deal becomes binding.

The arrangement is so unusual that the FTA hired an outside law firm to review it. The review found that the deal conforms to federal and state procurement laws, Dorn said.

In its approval letter to Rae, the FTA raised concerns about potential conflicts of interest among Virginia's private partners. Until early this spring, the West Group was one of the partners. The real estate firm, the biggest landowner in Tysons Corner, would have been involved in designing a rail line that would enrich its property values.

The West Group's decision to drop out of the private partnership reduced the potential for conflicts of interest, the letter said. But the FTA is requiring tougher guidelines for real estate negotiation between the state and its private partners.

Federal officials said they are also concerned that Virginia has never before built a billion-dollar rail project and lacks the experience necessary to manage it. The FTA has directed the state to hire a project manager with an extensive background in supervising such a project.

Opponents of the project say it will squander money. "Not only is it not a solution to traffic, but it's billions of dollars of taxpayers' money that are going to be wasted," said Gil Davis, an attorney for a group of Fairfax landowners who plan to file suit to stop the project.

The landowners, from the western part of the county, contend that Virginia officials violated state law when they selected rail to serve the Dulles corridor. They want the state to consider bus rapid transit, a relatively new and cheaper form of public transportation, in which buses run on a dedicated road and operate more like trains on rubber tires. A hearing in the case is scheduled today in Fairfax Circuit Court.