Metro said Sunday it is putting dozens of older rail cars back in service following a derailment, and starting Monday morning, riders should see signs of improvement on the Silver and Green lines.

Metro riders have experienced waits as long as 40 minutes since mid-October, after a single car on a Blue Line train slipped off the tracks outside the Arlington Cemetery station. The independent Washington Metrorail Safety Commission ordered the agency to pull its 7000-series cars, and the transit agency has since been relying on about 25 percent of its cars. They are working to bring older model 2000-, 3000- and 6000-series trains into the mix.

Starting Monday, Metro said in a news release Sunday, it will add seven more train sets — each set being six cars — increasing its total sets from 32 trains to 39.

“The added trains will allow Metro to provide Silver Line service to Largo Town Center instead of ending at Federal Center SW and improve Green Line service to every 20 minutes,” Metro said. Last week the Green Line was running every 30 to 40 minutes.

Trains will continue to come every 30 to 40 minutes on the Orange, Blue, Silver and Yellow lines. Red trains will come every 15 to 20 minutes.

The derailment was caused by a progressive defect that has caused some wheels to move outward, putting trains at risk for derailment. A National Transportation Safety Board investigation showed the defects had been uncovered during Metro inspections since at least 2017.

Metro board members have said they were not told of the defects before the derailment. Metro General Manager Paul J. Wiedefeld said he also didn’t know. The NTSB this month said the wheel assembly defect could have led to a catastrophic incident.

Scaled-back Metro service is expected to continue until mid-November.