Metro on Sunday night pulled more than half of its rail cars out of service hours before the Monday morning rush after an investigation discovered multiple axles out of compliance with manufacturer specifications.

The suspension of Metro’s latest train model — the 7000 series — comes after a National Transportation Safety Board investigation found the issue while probing a Blue Line derailment that occurred last week, according to the Washington Metrorail Safety Commission, an agency that Congress created to monitor Metro safety.

Metro is operating with about 40 trains Monday. The transit agency said in a statement it is “offering a basic service pattern on all lines of trains departing about every 30 minutes.” Metro on Monday urged commuters to use Metrobus as an alternative.

Metro apologized for the delays Monday, saying the 7000-series was removed “out of an abundance of caution.” In a tweet, the agency said, “We want the public to know we are committed to their safety and will continue to release updates as we receive them.”

The 7000 series includes 748 rail cars and makes up about 60 percent of Metro’s nearly 1,200-car fleet. It’s not known how long the lower service levels will be in effect.

A spokesman for the safety commission declined to comment but said next steps are laid out in the order signed by the commission suspending the rail car line. The order said Metro will create a plan to detect and assess the problem during an inspection and come up with a process that can safely return the cars to service.

The NTSB investigation found that the fourth axle of the car that derailed was out of compliance with 7000-series specifications, according to the order. Inspections of other cars not involved in the derailment found similar defects, the commission said.

NTSB spokeswoman Jennifer Gabris said Metro “is free to make any safety changes as a result of anything they have learned.” The federal agency planned a briefing Monday.

The Blue Line train carrying 187 passengers derailed Tuesday before it reached Arlington National Cemetery. The train operator reported a stuck holding brake before the train left Rosslyn.

The transit system’s 6000 series, which makes up about 15 percent of Metro’s cars, has been sidelined since November, when the agency pulled the trains out of service after two train separations on the Red Line over nearly two months. Metro discovered that some of the apparatus that joins the cars was restored with the wrong parts or tools, and it has only recently been returning a handful of the 184 cars to service.

Metro will now lean on its 1980s-era 2000 and 3000 series cars, of which the system has about 350.

Metro Board Chairman Paul C. Smedberg apologized for the inconvenience but said the suspension was a necessary precaution. D.C. Mayor Muriel E. Bowser (D) urged residents to plan for the additional travel time.

D.C. Public Schools said it expected Metro service levels to affect morning school operations, adding that late student arrivals will be marked as excused.

The most recent suspension comes as Metrorail ridership remains a third of pre-pandemic levels. But it also comes as the rail system was seeing increases in ridership over recent weeks.

Metro’s 7000-series cars, built by Kawasaki Rail at a plant in Nebraska, cost about $2 million each. They entered the system between 2015 and February 2020.