Metro’s limited train service will continue for the rest of November, officials said Friday, as the transit agency begins tests that it hopes will allow for the reinstatement of more than half of its fleet.

A rough outline for how long the nation’s third-largest transit agency expects to continue operating without most of its rail cars came after the Washington Metrorail Safety Commission on Friday approved Metro’s plan to test new inspection protocols. Metro officials hope the plan will allow the transit system’s 7000-series rail cars, suspended about three weeks ago, to return to service.

Metro has been operating with fewer than 45 trains — about a quarter of its rail cars — since Oct. 17, when the safety commission ordered all 748 of the 7000-series cars out of service. The cars make up about 60 percent of Metro’s fleet and had come under a National Transportation Safety Board investigation after the Oct. 12 derailment of a Blue Line train.

The investigation found a defect in some car wheels and axles that had appeared at least 50 times since 2017, making cars more prone to derailments.

The cause of the defect, in which wheels widen from fixed axles, remains under investigation, but the safety commission will allow Metro to put unaffected cars back into service — accompanied by an inspection process that would ensure cars showing signs of the defect are pulled from service. The safety commission is an independent agency that has overseen Metrorail safety since 2019.

Metro proposed inspecting wheels on each car every eight days. On Friday, the safety commission approved the testing plan. Metro must now test the new inspection schedule, which will take about two weeks, officials from Metro and the commission said.

In a statement, safety commission spokesman Max Smith said the agency had no technical objections to Metro’s plan after revisions were made, clearing the way for transit officials to “road test” two 7000-series trains in simulated service at Metro’s Greenbelt yard.

“The [safety commission] will closely monitor the testing activities,” Smith said.

If Metro’s engineering tests prove successful, that data will be part of a plan Metro will turn in to the safety commission that lays out how the agency would put the 7000-series cars back into service.

Metro said in a statement that testing will take a “couple of weeks” to complete. Through the end of the month, the agency said, Metro will continue to operate using its older-model cars, which include the 6000, 3000 and 2000 series.

Metro is pulling all of its available cars out of storage to help shorten waits, including 32 recently pulled from its Shady Grove yard. Waits between trains are about 15 minutes on the Red Line, 20 minutes on the Green Line and 30 minutes on the Orange, Blue and Silver lines, Metro said.

Metro on Friday had 43 trains in service, with seven being used to shorten waits during busy periods and to fill in for trains that break down or have other issues.

The NTSB-led investigation into the Blue Line derailment and the wheelset defect continued on Friday.

Jennifer Gabris, spokeswoman for the federal safety board, said the metal that makes up wheels and axles on the 7000 series is being tested at the NTSB Materials Lab. Investigators continue to conduct interviews and review evidence, she said.

Investigators have said the defect is unlikely to have originated during maintenance or repairs, and have focused on the construction of the rail cars and the pressing of the wheels to the axles.