Update: Fairfax County has approved its part of an agreement to help fund phase 2 of the Dulles Metro rail project.

“This strategy brings the cost of Phase II back down close to the original estimate and goes a long way to ensure this critical transportation project is completed,” Fairfax County Board Chairman Sharon Bulova said in a statement. “Dulles Rail remains my top transportation priority and I am glad to approve this strategy.”

 Original post: Final approval for building the second phase of the Metrorail line that will run to Dulles International Airport and Loudoun County, is expected to be signed by all involved parties by the end of the week.

On Tuesday, Fairfax County is expected to vote on whether to approve its portion of the deal for the “Silver Line” rail project.

Loudoun County has approved the deal, as has the Metropolitan Washington Airports Authority (MWAA), which oversees the construction of the Dulles rail project. Metro General Manager Richard Sarles could sign the transit agency’s version as early as Wednesday, a spokesman said.

The Dulles rail line is a $6 billion project that involves 23 miles of new Metro track.

The first phase of the project is under construction and runs through Tysons Corner to Reston. It is expected to open late next year, but contractors and officials for the airports authority have said it could be as much as $150 million over budget.

The second phase of the project will run from Wiehle Avenue to Dulles International Airport and to Loudoun County. How to pay for the second phase has been a long-running debate.

U.S. Transportation Secretary Ray LaHood stepped in to try to bring a deal to the table, and the parties reached an agreement in November. Each jurisdiction and agency needed to have its board approve the final details of the agreement.

Loudoun and Fairfax counties are seeking public-private partnerships to build stations and parking garages in their areas. Virginia is expected to put in $150 million for the project, and Dulles Toll users will fund a large chunk of the project’s cost.

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