Contractors have finished work on the last major element of the multibillion Silver Line rail project, officials said Wednesday, a milestone in the decades-long effort to extend Metrorail service to Dulles International Airport and into Loudoun County.

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       The rail yard was the last major hurdle for a project that has been plagued by multiple delays. The announcement from the Metropolitan Washington Airports Authority, which is overseeing construction of the rail line, clears the way for Metro to begin the evaluations needed before the rail line can begin carrying passengers.

       The announcement came less than a month after MWAA declared that work on the other major component of the Phase 2 extension, the rail line, was substantially complete.

       Silver Line extension to Dulles inches closer to completion after years of delay

       “The completion of the rail yard and maintenance facility is critical to completing the Metrorail Silver Line to Dulles Airport and beyond,” said John E. Potter, MWAA’s president and chief executive. “We are now one step closer to making that happen.”

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       It will be up to Metro to determine when passenger service on the rail line will begin.

       The rail yard and maintenance facility is being built by Hensel Phelps on 90 acres of airport property along Old Ox Road. It will provide storage for 184 rail cars. A separate contractor, Capital Rail Constructors — a joint venture between Bethesda-based Clark Construction Group and Kiewit Infrastructure Group — was responsible for building the rail line.

       While MWAA is overseeing construction, the project eventually will be handed off to Metro, which will manage and operate the rail line. Phase 2 of the Silver Line originally was scheduled to be completed in July 2018.

       Silver Line's second phase encounters bumps on the road to completion

       Metro previously estimated that it could take five months of testing and other preparations before passenger service can start. The Washington Metrorail Safety Commission, which is responsible for safety oversight of the rail system, also must verify that Metro has followed its safety certification process.

       The first phase of the Silver Line opened in July 2014 with five stations — four in Tysons and one in Reston. The second phase will have six stops, including one at Dulles Airport.

       The total cost of the first and second phases is $5.8 billion. Fairfax and Loudoun counties, as well as MWAA, are helping to pay for the project, but most of the bill is paid by Dulles Toll Road users.