A funding plan for Metro's Silver Line to Dulles Airport may have one flaw: It depends on toll road money from drivers who will no longer be driving, but actually using the Silver Line.

That was the concern raised this week by Fairfax County's Office of Financial and Program Audit. In response, the Board of Supervisors is expected at its Tuesday meeting to request an explanation from the Metropolitan Washington Airports Authority, who commissioned the 2010 study to support the funding plan in the first place.

And that, of course, could lead to more back and forth about hypothetical road use and train use depending on seemingly endless variables.

"Those things are so complex," said Supervisor John Foust, chairman of the audit committee. "It really illustrates why we'll need to work more closely with the federal and state governments to help pay the costs for phase two of the project."

In a stroke of better news, Foust said that the first phase of construction — from East Falls Church to the eastern edge of Reston — is going according to plan, save for being about 111 days behind its scheduled open in 2013.

"But the team believes there will be opportunities to make that up. It's not a given that it'll be late," he said.