There is no METRO RAIL service from Dulles, which is in Loudon County, VA and Downtown Washington, DC. Arriving into Dulles, you would most likely be taking a "Washington Flyer" cab or van. That is the cab company that has a monopoly on Dulles Airport. Although I fly out of Dulles a lot, I never use that cab service, drive myself, so I do not know what the prices are. There has been talk about building a light rail service from the city out to Dulles, but that would be light years away at this point. Now on the otherhand, Regan National has a METRO RAIL stop right there outside of the Delta Airlines ticket counter.