Phase 2 of the Silver Line, which will extend Metro to Dulles International Airport and into Loudoun County, is expected to open in 2020. The service might be operated by private contractors. (Katherine Frey/The Washington Post)

It’s official: When Metro opens the second phase of the Silver Line extension, tentatively scheduled for 2020, it’s likely that the six new stations, and potentially even some of the trains running along that segment, will be operated and maintained by private contractors.

On Tuesday, the transit agency issued a request for proposals from private companies willing to perform maintenance and operations on the line extension, which is under construction by the Metropolitan Washington Airports Authority. The extension will stretch from the Wiehle-Reston East station to Dulles International Airport and into eastern Loudoun County.

Metro has hinted for the past two years that its intention was to outsource the Silver Line service, suggesting that such a decision could save taxpayers millions of dollars in the long run. In January, the agency issued a “request for information” from potential contractors interested in the job.

Now, Metro says that hiring a private company to fill new Silver Line jobs, rather than adding to the ranks of unionized employees, will help control operating and maintenance costs, “including future pension costs, which have grown to unsustainable levels.”

Last week, a Government Accountability Office report concluded that Metro’s annual pension liabilities are unsustainable and jeopardize the future of the system.

Privatization may also save taxpayers money, at least in the short term. Though Metro is legally required to curb growth in its annual operating subsidy paid by the District, Maryland and Virginia, the year-to-year operating costs of the Silver Line extension are considered a bonus service — and mean that regional leaders will have to come up with the extra millions annually to pay for the new service.

Metro General Manager Paul J. Wiedefeld said the effort is intended to help the transit agency start “living within our means."

“Competitive contracting is one tool to hold down pension cost growth, while providing quality service for customers,” Wiedefeld said in a statement.

The plan is already getting pushback from Metro’s largest union, Amalgamated Transit Union Local 689, which represents the system’s train and bus operators. The union has long argued that outsourcing daily transit services results in poor service for riders and subpar maintenance of valuable infrastructure.

“Privatization is not a cost saving, but instead drives up fares, jeopardizes safety, results in service cuts, and fosters an environment for political corruption because it puts profits ahead of the riding public,” the union said in a statement Tuesday.

Additionally, the union argued that Metro cannot legally outsource Silver Line jobs, suggesting it plans to file a grievance with the transit agency.

But Metro officials say they are within their rights to use contractors for the new service. The agency has a four-year contract with Local 689 that bars it from outsourcing any existing jobs to the private sector. But because phase 2 of the Silver Line will require a host of new positions to maintain and operate the six new stations and 11-mile stretch of tracks, those jobs are free to be outsourced, Metro says.

The proposal would potentially be Metro’s most significant foray into privatization. Two months ago, the agency butted heads with ATU Local 689 after announcing that it planned to outsource bus maintenance and operations at a soon-to-open bus facility in Northern Virginia. That $89 million contract, Metro says, is expected to save the region’s taxpayers $15 million over the next five years.

“The proposal for the Silver Line extension will not result in a loss of jobs for Metro’s current workforce,” the agency said in its statement announcing the plans for privatization.

Phase 1 of the extension opened in July 2014, and included five new stations and 11.7 miles of track.

According to its announcement, Metro is looking for a company willing to perform a wide range of duties, including operation and maintenance of the “stations and facilities, including the rail cars assigned to the Silver Line, track and infrastructure, the Dulles Rail Yard, and all administrative functions necessary to support operations.”

Additionally, the agency says it is also potentially interested in hiring that company to operate the trains that run on that stretch of the Silver Line.

Metro expects to award the contract in the spring of 2019.

Correction: A previous version of this story misstated the amount that Metro stands to save with its forthcoming bus maintenance and operations contract. The $89 million contract is expected to save taxpayers $15 million over five years.