Metro riders can expect the current level of limited rail service to continue for three more months after the transit agency’s top official said Thursday that Metro wants to focus on finding the “root cause” of a defect that has sidelined more than half its rail cars since mid-October.

Metro’s 7000-series cars were briefly reinstated in December before a regulatory agency suspended them a second time after Metro failed to follow its own guidelines for safety inspections. The agency at the time was working to revise and streamline the guidelines, which required a daily inspection of the cars’ wheels.

The daily checks were meant to determine whether cars were showing signs of the defect, which causes wheels to widen out from their axle, but the task of measuring every wheelset proved too time-consuming for Metro’s staff, said Metro General Manager Paul J. Wiedefeld. He said Thursday he wants to give engineering and mechanical experts time to acquire technology to measure the wheelsets with a tool built into the track bed.

The delay is the latest setback in recent weeks as Metro has tried to return the troubled rail car series back to service. The agency had been focused on luring back riders during the coronavirus pandemic but instead has been forced to turn its attention to a derailment, a federal safety investigation and the loss of most of its fleet.

In the meantime, Wiedefeld said, Metro will continue to operate with older-model rail cars that have been called back into service from storage. The transit agency is also accelerating repairs and reconstructions on its 6000-series cars, which are coming back online after being taken out of service in November 2020 because of multiple train separations.

“We are going to redirect our efforts towards identifying and tackling the root cause of the derailment and take steps to better support more continuous wheel measurements by installing track bed technology,” Wiedefeld said in a statement.

The ongoing troubles have frustrated riders and government officials in the Washington region, where transit commuters made up 13 percent of the population before the pandemic, according to a census survey. The problem has been exacerbated in recent weeks as the omicron variant of the coronavirus has sickened more Metro workers than ever during the pandemic, creating a shortage of bus operators and forcing the transit agency to reduce bus service alongside rail service.

The National Transportation Safety Board (NTSB) uncovered the wheelset defect on 7000-series cars while investigating an Oct. 12 derailment of a Blue Line train that forced the evacuation of 187 passengers. Emergency inspections of all 748 cars found the defect in almost 20 cars, while investigators learned Metro technicians had known about the malfunction as early as 2017.

In response, the Washington Metrorail Safety Commission, an agency Congress created to monitor safety, suspended the cars until the transit agency came up with a plan to safely put the cars back into service. The commission approved Metro’s plan last month to inspect cars daily, but Wiedefeld said the “efforts to maintain and inspect trains” allowed for no more than five trains being put back into service each day.

The NTSB’s investigation is ongoing. Wiedefeld said Metro will work to determine the cause of the defect with the NTSB, consultant Transportation Technology Center and the safety commission.

Commission spokesman Max Smith said Metro and its experts are having frequent discussions.

“It is up to Metrorail to determine the amount of time needed to gather the information and tools it needs to develop and implement an effective plan, as required under our order, to provide for the safety of riders and workers,” Smith said in a statement.

Passenger wait times since the 7000 series was pulled from service — once 30 minutes or more — have been reduced to less than 10 minutes on all lines, Wiedefeld said, a contention that some riders have disputed. The rail system is carrying less than a third of the riders it was before the pandemic due to increased telework and the surge in coronavirus cases.

Metrobus, meanwhile, has been carrying about two-thirds as many customers, outpacing the much larger rail system. The bus system serves a higher proportion of customers who don’t have other options while D.C. schoolchildren rely on it as the city’s de facto school bus system.

Five D.C. Council members Wednesday urged Metro to restore normal bus service, especially after parents reported having to pick up children from bus stops when buses didn’t show. The five members of the council’s Committee on Transportation & the Environment signed a letter to Wiedefeld expressing concern about whether the agency can deliver efficient and safe transportation.

“This is not acceptable, nor is it sustainable,” the letter said. “This service change has forced residents who depend on reliable service, such as students and seniors, to wait in the cold and in inclement weather for interminable periods of time.”

Council members said the service reduction was “in part due to low vaccination and compliance with weekly COVID-19 testing protocols among Metrobus employees.” They proposed that Metro ease the burden on unvaccinated Metrobus operators and instead have drivers take tests before every shift, which council members said would reduce the number of operators held out of work for failing to test.

If Metro can’t restore normal service, the council members requested that Metro at least staff its reduced schedule more reliably and add several specific routes not being serviced under the current plan.

Metro in October made vaccinations a requirement — one of the few transit agencies to do so.

Wiedefeld responded to the letter, saying the number of employees failing to meet requirements was “minute.” On average, less than 2 percent of workers are not in compliance a day, Metro spokeswoman Sherri Ly said.

“It’s been a has been a very tough period for us, to be frank,” Wiedefeld said. “We have the most COVID cases that we’ve had since the pandemic. Our absenteeism rate is very high.”

Metro board member Tracy Hadden Loh said she has heard low vaccination rates being blamed for the shortage, but she and Wiedefeld pointed out that more than 85 percent of Metro employees are vaccinated.

“We’re all hopeful that this spike is going to come down very quickly, and I think we’re watching that across the globe,” he said. “If that occurs, we’ll bring back the service as quickly as we can.”