Fairfax County officials formally agreed Tuesday to the terms for a $403 million, low-interest federal loan they will use to help pay their share of construction costs for the second phase of the Silver Line.

The loan,  which comes from the U.S. Department of Transportation as part of the Transportation Infrastructure Finance and Innovation Act (TIFIA), will cover roughly 45 percent of the county’s estimated share of $915 million. Under the terms of the loan, they money does not have to be paid back until five years after the completion of the $5.6 billion project. Phase 2 of the Silver Line will extend service to Dulles Airport and into Loudoun County, is expected to cost $2.7 billion and be completed in 2018.

The first phase of the rail line opened in July. The rail line was six months late and, with bills still being totaled, could very likely exceed its $2.9 billion price tag.  Still, the rail line has proved popular with the public. Metro officials said ridership is strong, even though Metro ridership overall is down to 2005 levels. The first phase of the rail line includes four stops in Tysons Corner and one in Reston at Wiehle Avenue.

“With Phase 1 on track to meet or even exceed ridership projections, I am very pleased to have approved low-interest federal financing for Phase 2,” said Sharon Bulova, chairwoman of the Fairfax County Board of Supervisors, in a news release announcing the board’s action.  “The funds made available through the TIFIA loan keep the project on target for completion in 2018 and reduce costs for toll road users and taxpayers. The Silver Line is vital to the economic growth of Fairfax County, the region and the Commonwealth, and I am proud of the work we’ve done to make it a reality.”

Fairfax officials will use money from the Dulles Rail Phase 2 Transportation Improvement District and Commercial and Industrial Tax Fund to repay the loan. The county use $218.2 million from this voluntary tax district and $185.1 million in commercial and industrial taxes.

The federal money was key to a 2011 financing deal to build the rail line’s second phase. In total, Fairfax County, Loudoun County and the Metropolitan Washington Airports Authority received nearly  $1.9 billion in TIFIA loan money. It was the largest loan in the history of the TIFIA program. The money also helped stave off further increases for users of the Dulles Toll Road, who are financing the bulk of the Silver Line’s costs. The federal dollars, combined with $300 million from the state of Virginia, likely mean that there will be no more toll increases until 2018, MWAA officials said. Tolls on the 13.4-mile road have increased for five straight years.

As part of the Silver Line funding agreement,  Fairfax County will pay 16.1 percent by Fairfax County, Loudoun County will pay 4.8 percent and MWAA will pay 4.1 percent. The remaining 75 percent comes from Dulles Toll Road users.

The line’s second phase will extend from Wiehle Avenue in Fairfax County to Route 772 in Loudoun County.