Reduced Metrorail service will continue through Dec. 31, the Washington Metropolitan Area Transit Authority announced Monday.

WMATA officials will make “incremental” service improvements through December as they wait for parts to arrive for older railcars. The continued delays will last for the duration of the University’s U-Pass trial period taking place next month.

The service delays follow a Blue Line train derailment last month, which led to safety concerns about Metro’s 7000-series fleet. Metro officials said there is “no timeline established” to return the railcars to service as they test new inspection protocols.

“This is a monumental undertaking that is being performed on parallel tracks to ensure that we have as little down time as possible between testing and remobilizing the fleet,” Metro General Manager Paul Wiedefeld said in a release. “We are intentionally not setting deadlines so that safety and good data drive our decisions, but we are mindful that customers want the best service we can provide as soon as we can deliver it, and we are committed to building back up in phases.”

Metro said officials are working to put older trains in service to decrease wait times, but 6000-series railcars are awaiting parts due to global supply chain challenges.

“As we get more parts, we will return more of the 6000-series railcars to service for our customers during December,” Wiedefeld said. “While we know service is not as frequent as customers would prefer, we will add each train as it becomes available to help incrementally improve service reliability and frequency.”