The second phase of the new Silver Line to Dulles Airport and Loudoun County is expected to cost $2.7 billion — slightly less than previous cost estimates for the project.

The Metropolitan Washington Airports Authority (MWAA) released the new figure Tuesday, as it prepares to issue a request for proposals on bids to build the second part of the 23-mile stretch of new Metrorail line.

A westward facing view of the Dulles Toll Road from the Wiehle Ave. overpass where the Silver Line of the Metro will be extended to Dulles Airport. (Jahi Chikwendiu/The Washington Post)

Construction of the second phase is expected to start in January, and take five years.

Last year, Transportation Secretary Ray LaHood personally helped negotiate a deal in getting the second phase started after debate arose over whether build an underground or above-ground station at Dulles International Airport. The MWAA board ultimately decided on an above-ground station.

An escalator awaits installation at the Wiehle Avenue station on the first phase of Metro’s new Silver Line in September 2011. (Michael D. Bolden/The Washington Post)

U.S. Transportation Secretary Ray LaHood. (Christopher Powers/BLOOMBERG)

The new estimate is slightly lower because the design has been more developed, which allowed engineers to make better estimates, MWAA officials said.

The new, $2.7 billion cost assumes that Loudoun and Fairfax counties pay for construction of five parking garages and the Route 28 station. If the two counties don’t pay for that portion, the cost of the project would rise to $3.1 billion.

MWAA spent $47 million for a joint venture that included AECOM and Parsons Brinkenhoff to do the latest preliminary engineering and cost estimate. It hired another company — McDonough, Bolyard and Peck — to do another cost estimate. That firm estimated the second phase of the Silver Line would cost $3.3 billion.

Patrick A. Nowakowski, the executive director of the Dulles rail project, said the reason for the difference is the two firms used different assumptions for the impact of inflation.

Fairfax and Loudoun counties now have until June 4 to decide whether they will participate in the deal. Officials in Loudoun had expressed concern about the project’s cost.

Sharon Bulova, chairwoman of the Fairfax County Board of Supervisors, said Tuesday, “the figures look pretty much in line with what we’ve been anticipating.” She said Fairfax has advertised to try to find a developer to do a public/private partnership to develop their portion of the parking garages.

Bulova said public hearings would be held over the next month for the public to “weigh in and learn more” about the second phase of the Silver Line.

The second phase of the project has raised controversy among Virginia politicians who are worried about a project labor agreement and have pushed Gov. Bob McDonnell (R) to withhold the state’s $150 million contribution toward the project’s cost.

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