Metrorail riders should expect continued delays through the rest of the week after more than half of the city’s railcar fleet was pulled Monday for an investigation into last week’s train derailment, Metro officials announced Monday.

Trains will depart every 15 minutes on the Red Line and every 30 minutes on all other lines starting Tuesday while the National Transportation Safety Board continues inspecting the derailment, which was linked to a wheel and axle assembly malfunction, according to a WMATA release. NTSB Chair Jennifer Homendy said at a press conference Monday that WMATA knew of at least 31 similar equipment failures, dating back to 2017, and found 21 more issues in railcar inspections since Friday.

“We are fortunate that no fatalities or serious injuries occurred as a result of any of these derailments,” she said. “But the potential for fatalities and serious injuries was significant – this could have resulted in a catastrophic event.”

The Washington Metrorail Safety Commission ordered WMATA to pull all 7000-series railcars, about 60 percent of its fleet, from operations Monday, leading to service delays at all stations. Homendy said WMATA officials are inspecting the railcars for more failures and that it is unclear when the railcars could return to service.

Homendy said other transit agencies who operate vehicles manufactured by Kawasaki – the Japanese company which created the 7000-series cars – should inspect their cars to ensure there is not a “systemic” issue with Kawasaki trains throughout the country.

Silver Line trains will only run between the Wiehle-Reston East and Federal Center SW stops starting Tuesday, WMATA said.

“I want to assure our customers that their safety is driving every decision being made,” Metro General Manager Paul Wiedefeld said in the release. “We apologize for the reduced service, and ask for our customers’ continued patience and support as we work to get Metro back to normal operations.”