Traffic is seen on Route 123 near the Tysons Corner Station of the Silver Line rail extension on Tuesday. (Jahi Chikwendiu/The Washington Post)

The Silver Line project will receive a low-interest, long-term federal loan of nearly $2 billion to help pay for the second phase of the project that will extend the new line to Dulles International Airport and farther into Loudoun County.

Transportation Secretary Anthony Foxx has signed the Transportation Infrastructure Finance and Innovation Act loan, the largest in the history of the federal program, Virginia’s congressional delegation announced Thursday.

The U.S. Department of Transportation’s TIFIA program gives credit assistance to help finance major surface-transportation projects. The Silver Line is one of the largest infrastructure projects being built in the United States.

The loan, given to the Metropolitan Washington Airports Authority and its funding partners, Fairfax and Loudoun counties, represents about one-third of the rail line’s cost. MWAA is responsible for managing construction of the 23.1-mile rail project, which will be operated by Metro.

The $1.9 billion loan also may help ease some of the pressure on those who use the Dulles Toll Road, who will pay the bulk of the rail line’s $5.6 billion price tag.

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.

“Today’s final approval of the TIFIA loan is great news for the future economic growth of Northern Virginia,” Sen. Mark R. Warner (D) said in a statement. “Our bipartisan congressional delegation has been unified in pushing to get this critical piece of financing, which will provide welcome and meaningful relief for commuters using the Dulles Toll Road. This is a huge step forward for a project that has been decades in the making.”

Drivers generally pay $3.50 per trip on the toll road. Without the loan and money from the state, tolls would have increased to $4.50 in 2015, MWAA officials said.

“This will have an impact,” said Rep. Gerald E. Connolly (D-Va.). “It freezes toll rates for five years or more and . . . puts significant downward pressure on the need for toll increases” in other years.

But while the loan will slow toll increases, it will not halt them indefinitely. According to current MWAA projections, tolls will increase to $4.50 in 2019 and to $6 in 2023. By 2043, drivers could expect to pay $11.25 to travel the toll road.

Rob Whitfield, of the Dulles Corridor Users group, was less than enthusiastic about the news. He continues to believe that it’s unfair for toll road users to have to pay the majority of the rail line’s costs.

“The Dulles Rail project financing scheme is a massive unconstitutional tax,” Whitfield said.

Still, Rep. Frank Wolf (R-Va.) said that Virginians should feel good about the federal government’s approval of the loan package. The Department of Transportation “believes this is one of the most important projects in the country,” he said.

After months of delays, the first phase of the Silver Line is expected to open this summer. It will have four stops in Tysons Corner and one in Reston, at Wiehle Avenue. Preliminary construction on the second phase of the rail line, which will have six stops, including one at Dulles International Airport, has already begun.

The Department of Transportation’s $1.9 billion “loan guarantee is a much-needed federal investment in Phase 2 of the Dulles Metrorail Project that brings us one step closer to securing an environmentally friendly, transit oriented development along the Toll Road,” Rep. James P. Moran (D-Va.) said. “Paving our way out of the worst traffic in the nation is impossible. This extension is critical to reducing congestion, commuting time and gas consumption.”

For the first phase, the Department of Transportation provided $900 mil­lion but made it clear that federal support for the second phase would come in the form of loans, not grants.

TIFIA provides direct loans, loan guarantees and standby lines of credit to help finance significant regional and national transportation projects. The Chicago Transportation Authority recently received $79 million in TIFIA funding to renovate the 95th Street Rail and Bus terminal and a $288 million loan to help fund an intermodal facility at O’Hare International Airport.

“With today’s announcement, the last bar has been cleared to move forward on [Phase 2] of the Silver Line, which will improve the daily lives of Northern Virginia commuters and spur economic development throughout the region,” said Sen. Timothy M. Kaine (D-Va.), who as governor worked to secure federal funding for Phase 1. “This has been a tremendous bipartisan effort to bring this project home, and I believe it will be well worth the wait.”

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