Metro riders saw a familiar sight Monday as a pair of 7000-series trains made their way through the system during service hours for the first time since Oct. 17, when the series was sidelined for a safety issue that has caused a severe train shortage.

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       The trains are stopping at stations but not picking up passengers, part of a test to get all 748 of the series’ cars reinstated.

       Train doors are marked with stickers that say, “No passengers, test train.” Inside, heavy boxes sit in rail car aisles meant to simulate the weight of passengers. Metro officials have indicated testing could run about two weeks. If successful, the data will be part of a plan that transit officials will submit to the Washington Metrorail Safety Commission to request reinstatement of a series that makes up about 60 percent of Metro’s fleet.

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       When that could happen, however, isn’t known.

       “We haven’t put a time frame on any of this,” Metro spokeswoman Kristie Swink Benson said. “Again, it’s about safety. It’s about doing this right.”

       Metro submits testing plan aimed at restoring suspended rail cars

       The safety commission, an independent government agency that oversees Metrorail safety, ordered Metro to pull all 7000-series trains from service after a National Transportation Safety Board investigation into an Oct. 12 Blue Line derailment uncovered a defect. The malfunction pushes wheels outward from fixed axles, putting them at risk for derailments. Investigators said the defect appeared only a handful of times from 2017 until this year, when nearly 40 of the roughly 50 cases were discovered.

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       Safety inspectors, engineers and investigators said they are puzzled by the malfunction, which they described as unusual and progressive, surfacing only after a period of use. The NTSB has sent metal from the wheelset of the derailed train to its lab, and the cars’ manufacturer, Kawasaki Rail Car, and wheelset assembler ORX are part of the federal safety board’s team searching for a cause.

       The safety commission has allowed Metro to return unaffected rail cars back into its operating fleet, provided the transit system proves it can do so safely. Metro has proposed increasing the frequency of wheelset inspections from 90 days to eight days, an interval officials believe allow for the detection of the earliest signs of the defect.

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       The commission has signed off on testing that plan, which is the test that is underway.

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       Metro officials said the series won’t be reinstated at least before the end of the month.

       “We are improving our headways,” Swink Benson said. “I think you can see that week over week, and we continue to look at ways to improve service for our customers.”

       Metro to continue operating without most of its fleet until December

       The suspension of cars has forced Metro to pull every available older model car from storage to keep the system operating on a basic schedule. Transit officials said they have about 50 trains available without the 7000 series.

       On Monday, Swink Benson said Metro reduced waits to every 12 minutes from 15 minutes on the Red Line, while trains ran every 20 minutes on the Green and Yellow Line — a shorter wait time that transit agency officials hope will continue amid the shortage. Blue, Orange and Silver line trains continue to operate every 30 minutes.

       Transfer stations that serve multiple lines saw the shortest waits, at about 10 to 15 minutes.