WASHINGTON (AP) — Commuters in the Washington area will face longer waits for Metro trains through at least the end of the month and probably longer, as more than half the fleet of train cars will remain out of service over safety issues.

Metro General Manager Paul Wiedefeld told reporters Friday there was no timeline for the return of the transit authority's 7000-series train cars, which were abruptly pulled from service this week after a derailing revealed a chronic problem with the wheels and axles. The 748 cars are the newest and comprise more than 60% of the fleet.

The derailing is under investigation by the National Transportation Safety Board, and Wiedefeld said he couldn't comment on it while the investigation was ongoing.

In the meantime, Wiedefeld said Metro engineers were working to bring older retired cars back into service. But he said the older cars were undergoing rigorous inspections and he didn't expect them back in service until early November. Until then, riders can expect longer delays between trains across the system, whose six lines crisscross the city and stretch deep into the Maryland and Virginia suburbs.

The transit authority currently has 268 rail cars in service. There are 180 cars from the older 6000-series trains that may be brought back if they pass inspections. Currently only 22 of them are available for service.

The suspension of the bulk of Washington's Metro fleet was prompted when a train car slipped off the tracks on the Metro’s Blue Line near Arlington National Cemetery on Oct. 12. The car had apparently derailed once and then re-connected with the rails by itself, before derailing a second time. Some passengers were trapped in a tunnel in a dark train car and had to be evacuated on foot.

After the derailing, the NTSB revealed that the Kawasaki-made 7000-series trains had been suffering an escalating series of incidents due to a design flaw that caused the wheels to spread too wide on the axles, allowing the carriage to slip off the tracks. The issue had been apparent to the Washington Metropolitan Area Transit Authority, which oversees the regional D.C. transport system, since 2017, but neither NTSB nor the WMATA board had been informed, said NTSB Chair Jennifer Homendy.

The system experienced 18 separate incidents with the wheels in 2021 alone, Homendy said, and inspections revealed similar problems with an additional 21 rail cars.

Wiedefeld said his agency was now working to “inspect every wheel” on the 7000-series cars, proceeding in consultation with both Kawasaki and the NTSB. He acknowledged that WMATA could be blamed for failing to address the issue or inform NTSB or its own board of the problem.

“I understand that erodes some confidence," Wiedefeld said. “We're not hiding anything. We want to be as transparent as possible.”

Paul Smedberg, the Metro board chair, told reporters the board would “reserve judgment” on whether Wiedefeld should have brought the issue to their attention earlier.

“We're behind the entire team looking to get service back safely,” he said. “We've got to make sure we get this right.”

In an apparently unrelated incident, train service was suspended Friday afternoon on the Green and Yellow lines downtown and passengers were evacuated from a disabled train over a suspected braking problem. There were no reported injuries.