A week after Virginia officials decided not to tunnel under Tysons, planners for a $4 billion extension of Metrorail are seeking to make up months of lost time and meet a completion deadline only a few years off.

The Dulles Corridor Metrorail Project, a $4 billion plan to construct 23 miles of track to connect D.C. with Dulles Airport, has a first-phase completion date of 2012, and a final completion in 2015. The project saw about five months of delay while officials studied whether the 4-mile portion of the track running through Tysons Corner could be built underground.