When the $6.5 billion Dulles Metrorail project — the Silver Line — is complete in 2016, the main purpose won't be to serve passengers headed to the airport, but to carry workers headed for terrestrial jobs.

"The main thing to understand is that the primary destination is not Dulles. It's Tysons Corner," said Zachary Schrag, an associate professor of history at George Mason University, who wrote a book on the history of the Washington Metro system.

The basic idea, he said, is to provide rail transportation for the 100,000 people who work in Tysons Corner, VA, which will have four stops along the Silver Line.

As Stephen Fuller, the director of George Mason University's Center of Regional Analysis, pointed out, Fairfax County, VA has more jobs than the District of Columbia.

"That's the future of this region, and it needs to be endowed with the best transportation," he said.

The costs to build the new Metro line are immense, but the costs of not building the extension will be greater in the long run, Fuller said.

"The rail becomes more important than just moving people between the end points. There's going to be a lot of local traffic on it," Fuller said. "It's a very long line. The impact on real estate values and the impact on the economy is going to pay (the project cost) back many times over."

The project, managed by the Metropolitan Washington Airports Authority, will add 23 miles to the 106-mile Metro system. Construction on an 11.7-mile section, from East Falls Church to Wiehle Avenue in Reston, VA is expected to be finished by 2013.

When passengers can climb aboard, though, isn't clear. After construction, the Washington Metropolitan Area Transit Authority will begin extensive safety tests.

But an end to the "megaproject" is nearly in sight — which is significant considering discussions to build a rapid transit line to Dulles began nearly 50 years ago.

"For a long time it was just something that was out there. It was a dream. It was a vision," said Marcia McAllister, a spokeswoman for the airports authority. "The airport itself opened in 1962 and Dulles was just way out there. Right after that, literally, within days, talk began about building a road and a rail line eventually to connect Dulles to downtown."