WASHINGTON (AP) — Washington’s regional Metro system abruptly pulled more than half its fleet of trains from service early Monday morning over a lingering problem with the wheels and axles that caused a dramatic derailing last week. The ruling promises to complicate daily travel and commutes for thousands of riders for an unspecified length of time while the National Transportation and Safety Board investigates the issue.

The Metro authority’s safety commission ordered the withdrawal of the entire 7000-series line of trains at dawn. The Kawasaki-made 7000-series are the newest set of trains in service and the 748 cars comprise about 60% of the fleet.

NTSB Chair Jennifer Homendy told reporters Monday that a design flaw had been identified that caused the trains’ wheels to spread too wide on the axles, allowing the carriage to slip off the tracks.

“We’re at the preliminary stage of our investigation — just trying to collect data and information,” Homendy said. “This could have resulted in a catastrophic event.”

The wheel issue is being blamed for a incident last week in which a train car slipped off the tracks on the Metro’s blue line near Arlington Cemetery. Homendy said the car had apparently derailed once and then re-connected with the rails by itself, before derailing a second time. Some passengers were trapped in a tunnel in a dark train car and had to be evacuated on foot.

The safety ruling had already snarled commutes across the nation’s capital and the intertwined communities of northern Virginia and southern Maryland. Passengers on social media reported widespread delays with commuters waiting up to 45 minutes between trains and crowding chaotically into whatever space was available.

The problem comes as WMATA is working to attract more riders, after the numbers plummeted due to the COVID-19 pandemic. Overall rider numbers remain at a fraction of pre-pandemic levels, but are expected to increase steadily as offices reopen and tourists return to Washington.

In remains to be seen whether the axle issue spreads to other regional commuter systems. Kawasaki supplies trains to the MTA in New York City, among other systems, and Homendy said the NTSB “may at some point” issue a recommendation for inspections of all similar train cars around the country.

“If you are a transit agency operating in the United States and you’re listening, make sure you are checking your cars as well,” she said.

The incident promises to shine a potentially harsh light on WMATA, the regional transit authority which suffered a string of embarrassing and dangerous derailments and track fires, several years ago but claimed to have addressed its issues.

Homendy revealed Monday that WMATA had been aware of the potential problem since 2017, but hadn’t informed the NTSB, and that minor incidents with the wheels had been escalating since then. She said WMATA was aware of 18 such failures in 2021, and the current round of emergency inspections revealed an additional 21 failures — for a total of 39 this year alone. Homendy said the inspections were ongoing that “that number could go up” in the coming days.

The local Metro authority has not commented on this issue, other than announcements of reduced service and expected delays.