The decades-long effort to extend Metrorail service to Dulles International Airport and into Loudoun County moved another step closer to reality on Thursday.

The Metropolitan Washington Airports Authority said work on the rail line is “substantially complete,” a designation indicating that portion of the project is ready for Metro to begin testing. Construction continues on a second element of the project — a rail yard near the airport — and it wasn’t clear Thursday when the extension might open for customers.

The announcement is a major step forward for the $2.8 billion second phase of the Silver Line, which originally was scheduled to be completed in July 2018. MWAA had announced in March it would reach substantial completion by Labor Day, only to backtrack on the date — the latest in a string of delays and cost overruns that have plagued the project for years.

“This is a significant step toward completing the 11.5-mile extension that will provide rail service for residents in Reston, Herndon and eastern Loudoun County and give Metro riders direct access to Dulles Airport,” Jack Potter, MWAA’s president and chief executive, said in a statement.

Despite Thursday’s announcement, it’s not known when rail customers could begin boarding trains. MWAA is overseeing the project’s construction, but Metro eventually will operate the line.

Metro can begin some testing, but other pre-service evaluations can’t start until work on the rail yard is complete. According to the rail yard project’s most recent monthly update, the estimated timeline for completion is February 2022.

Metro said in a statement it “won’t take ownership” of the project until both elements are complete, but said testing could begin. Metro’s board must vote to officially take control of the rail line.

“Metro looks forward to entering the next phase of the project, during which we will perform hundreds of tests to ensure the extension can be operated safely and reliably before the Board accepts ownership and sets an opening date,” the transit agency said in a statement.

When testing is complete, Metro will begin about 90 days of training that will include service simulations and emergency drills. Afterward, it will set a date for passenger service to begin.

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

The rail line’s second phase is being built by separate contractors. Capital Rail Constructors, a joint venture between Bethesda-based Clark Construction Group and Kiewit Infrastructure Group, was awarded the $1.18 billion contract to build the rail line extension in 2013. One year later, Colorado-based Hensel Phelps won a $249.3 million contract to build the rail yard.

In a statement Thursday, Keith Couch, project director at Capital Rail Constructors, called the project a “transformational piece of infrastructure for the region.”

Delays have added more than $300 million to the project’s cost, which is expected to be covered by a $500 million contingency fund. The total cost of the first and second phases of the Silver Line is $5.8 billion.

The Silver Line is funded by the state of Virginia, the federal government, and Loudoun and Fairfax counties, but the bulk of its costs are paid by Dulles Toll Road users.

Virginia officials have long pushed to extend Metrorail service to Dulles and into Loudoun County, viewing it as critical to fostering development along the Dulles Toll Road corridor. Many of the soon-to-open Metro stations already are home to developments that include apartments, shops, restaurants and office space.

The first phase of the Silver Line, built by Bechtel, included five stations — four in Tysons and one in Reston — but also was dogged by construction issues. It opened in July 2014, six months behind schedule and more than $220 million over budget.

Construction on the project’s second phase, which includes six stops, began that same year and officials had hoped the work would be completed by mid-2018. Work on the rail yard was expected to be wrapped up by December that year.

But almost from the beginning the second phase ran into problems. MWAA’s decision to incorporate new requirements for storm water management delayed the rail portion of the project by 13 months and added at least $137 million to the cost. Other issues surfaced at the rail yard, delaying efforts to complete the project.

In 2018, a whistleblower filed a lawsuit alleging that a subcontractor falsified information about the quality of the concrete panels installed at five of the six stations. One person pleaded guilty in connection with the case, and the company eventually settled the suit for $1 million.

Similarly, numerous deficiencies were found with the rail yard, including cracks in concrete panels and platforms that had to be rebuilt because they were the wrong dimensions.

In a statement, Rep. Gerald E. Connolly (D-Va.), a Metro advocate and chairman of the House Oversight and Reform subcommittee on government operations, where Metro faces an annual hearing, acknowledged the obstacles to building a rail line to Dulles, but said the payoff will be a corridor that will be the “economic engine of the Commonwealth.”

“It is really hard to do big things, and this one has been 60 years in the making,” said Connolly, a longtime supporter of the project. “I am going to be a dog with a bone to get this project to completion and ensure riders have the safe and reliable transit service to Dulles our region first dreamt of in 1962.”