The leaders of the Senate committee that oversees transit asked Metro on Friday to turn over a detailed timeline of its efforts to address a defect that ultimately led to a derailment this fall and the sidelining of the agency’s flagship series of rail cars.

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       Sens. Sherrod Brown (D-Ohio) and Patrick J. Toomey (R-Pa.), the chairman and ranking GOP member on the Banking, Housing and Urban Affairs Committee, said in a letter that they want to understand what steps Metro and the manufacturers of the cars and wheel parts took after problems with wheels shifting on their axles first were identified.

       The committee is responsible for federal transit policy, and the senators said they were seeking the information “in furtherance of our oversight responsibilities.” It’s the latest probe into a derailment that has led to bare-bones Metro service for two months at a time when Metro had been planning for more workers to return offices.

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       Metro spokeswoman Sherri Ly said Friday the agency was reviewing the letter and would provide a response to lawmakers.

       The problems with the wheel assembly caused a train operating on the Blue Line to derail in October, leading safety regulators to order all 748 of the agency’s 7000-series cars be taken out of service. Emergency inspections revealed the defect, which involves wheels shifting outward, and Metro found the defect had affected 20 axles and appeared to get progressively worse over time.

       Wheels of derailed Metro car had moved far beyond agency’s safety limits, NTSB investigation shows

       Metro has said it first detected a problem in 2017, but that it initially appeared to be limited in scope and was working with manufacturer Kawasaki Rail Car to address it. But safety regulators were not told of the defect, and Metro has said little about how widely known the problem was within its upper management. Experts say issues affecting the spacing of wheels are a rare and serious safety risk.

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       The distance between the wheels is supposed to be fixed to within 1/16 of an inch, but they had shifted two inches on the derailed car, according to the National Transportation Safety Board.

       The NTSB, which is investigating the derailment, controls what information the subjects of its probes can disclose to the public, limiting what Metro has shared. But NTSB spokesman Keith Holloway said the board’s rules don’t prevent Metro from responding to inquiries from Congress.

       The Senate committee’s inquiry, also backed by Sens. Mark R. Warner (D-Va.) and Chris Van Hollen (D-Md.), could shed more light on what Metro officials knew and when they knew it. The committee asked for a response by Jan. 7.

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       The senators asked for descriptions of Metro’s interactions with its suppliers and any consultants, and for any analyses of safety risks posed by the defective wheel parts conducted by Metro employees.

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       In addition to investigations by the committee and the NTSB, the Office of the Inspector General for the U.S. Department of Transportation and its counterpart within Metro are conducting a joint investigation to determine why the malfunctions were not resolved sooner.

       Inspectors general investigate Metro over not reporting rail car defects

       The scope of that investigation centers on who knew about wheelset defects discovered more than 50 times over four years and what those with knowledge did with the information. Investigators are also searching for breakdowns in communication and notification.

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       The loss of the cars, which make up 60 percent of Metro’s fleet, has the agency facing its worst crisis in at least half a decade. Metro has tested 7000-series cars weighted with boxes to determine how often they would need to be inspected to safely return to service.

       On Thursday, the agency laid out plans to gradually reintroduce the cars and increase service levels in the system. The plan would have to be approved by the Washington Metrorail Safety Commission, and it’s not clear when it might be put into practice.