Metro’s troubled 6000 series trains are slowly returning to service after being sidelined for 10 months following a string of detachments and the rebuilding of the apparatus that connects rail cars.

Transit officials say they hope to return all 184 of the rail cars to the agency’s roughly 1,200-car fleet over the next three months, bringing back 15 percent of Metro’s stock in time for the planned opening of the Silver Line extension in 2022. It also comes as Metro is gearing up for an increase in ridership that has been slow to materialize during the pandemic.

“Our goal is to return the entire 6000-series fleet to service by the end of the year, dependent on a number of factors, including findings from our inspections,” Metro spokesman Ian Jannetta said.

Entering service in 2006, Metro’s 6000 series is the transit agency’s sixth model of rail cars custom-built for the system. Constructed by French multinational company Alstom Transportation at a cost of $378 million, the cars were taken out of service Nov. 24, 2020, the day a train had separated on the Red Line for the second time in two months.

In that more recent case, a train carrying 12 passengers detached between the Wheaton and Glenmont stations in Montgomery County. It followed an incident on Oct. 9, when two rail cars separated outside Union Station, stranding 108 passengers for nearly two hours. No one was injured in either incident, although two passengers were treated at the scene in October for chest pains and breathing difficulties.

The two incidents came after another 6000 series train separated while in service in August 2018 near the McLean station. None of the roughly 70 passengers was injured.

Investigations by Metro and the Washington Metrorail Safety Commission, the independent agency created by Congress to oversee Metrorail safety, found loose bolts and connections in car couplers, which work like latches at the end of rail cars to hold trains together. Other trains that were inspected after the 6000 series was suspended had incorrect bolts and hardware, investigators have said.

The coupler involved in the 2018 separation had a bolt that was the wrong size, Metro Chief Safety Officer Theresa Impastato said last year.

Investigators traced problems to a reconditioning process the couplers had undergone in recent years. Safety commission investigative reports from the two incidents last fall, as well as a recent audit of rail car maintenance, noted similar conclusions. They cited Metro for not following a systematic safety certification process with quality-control checks, documentation and record-keeping that could have prevented the errors.

“The probable cause of this pull-apart was a lack of and incomplete inspection, maintenance and overhaul procedures, including a lack of processes to ensure clamping, a failure to follow documented review and approval processes, and a failure to fully learn from prior events to implement systemic changes and reviews,” said the safety commission report on the Nov. 24 detachment.

Since the suspension, Jannetta said, Metro has put the cars’ couplers through an inspection process that includes an evaluation with consultation from the manufacturer “to assure that all couplers are free from defects.” The safety commission, he said, has been involved in the inspections and in how Metro returns cars to service.

The first 6000 series cars went back into service during the last week of September. Metro will review engineering and other reports on their performance, which will be shared with the safety commission.

Metro still needs to submit corrective action plans for issues the commission’s rail car maintenance audit raised earlier this year. Those include the need for a more effective safety certification process, which involves creating a system of guidelines, checks, documentation and record keeping during restoration work that can include the rebuilding of an entire car or replacement of worn parts.

“We’ve been in close communication with [Metro’s] rail car departments,” said Max Smith, spokesman for the safety commission. “The [commission] is closely following all of these tests, but [Metro] is still developing corrective action plan proposals required under the rail car audit, which are due in the next two weeks.”

Before the pandemic, Metro officials raised the possibility that the 6000 series could be retired early and replaced with the next model of rail car, the 8000 series. In September, Metro selected Hitachi Rail to build 256 cars with the option to build as many as 800 — part of a $2.2 billion contract that includes the creation of an assembly plant in the Washington region.

The initial order, not expected to begin arriving until at least 2024, will replace the transit agency’s remaining 2000 and 3000 series cars, which are nearing the end of their 40-year life span.

The rail car adjustments are happening alongside changes in ridership spurred by the pandemic. Rising ridership in 2019 gave way to steep losses of passengers and fare revenue. While rail riders are slowly returning — weekday ridership passed 200,000 trips on Sept. 24 for the first time since mid-March 2020 — it remains about one-third of pre-pandemic levels.

This fiscal year, Metro is projecting to take in less than one-fourth of pre-pandemic passenger fare revenue. More than $760 million in federal coronavirus aid is helping to make up for the losses.

Metro also will face increased operating costs when it opens the long-delayed $5.8 billion Silver Line extension, possibly during the first half of next year, according to estimates from Metro, the contractor and the Metropolitan Washington Airports Authority, which is overseeing the project. The nearly 11-mile extension would connect rail service from the Wiehle-Reston East Metro station to Dulles International Airport and through to Ashburn.

Metro will need to devote about 40 trains to operate the extension, Jannetta said, which includes six new stations in Fairfax and Loudoun counties.

The transit agency could see an infusion to support capital costs in the Senate’s bipartisan infrastructure bill. But continued low ridership and low revenue could lessen Metro’s need for them.

Jannetta said future purchases of 8000 series trains beyond the initial contract for 256 cars depend on annual budget decisions, which will take future ridership levels into account. “The 6000-series is an important part of our fleet, and we have confidence in its safety and reliability as it reenters service,” Jannetta said.