More than half of Washington‘s regional metro system was abruptly halted on Monday morning over a suspected problem with the wheels and axels.

The National Transportation Safety Board is currently probing the matter and is likely to disrupt the daily schedules of thousands of commuters who rely on the train system.

The Metro authority’s safety commission ordered the withdrawal of the entire 7000-series line of trains overnight. The Kawasaki-made 7000—series are the newest set of trains in service and the 748 cars comprise about 60% of the fleet, according to reports from Associated Press.

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Jennifer Homendy, the chair of the National Transportation Safety Board, explained that the carriage could slip off the tracks due to a newly identified design flaw that spread the wheels too wide on the axels.

Homendy said, “We are at the preliminary stage of our investigation — just trying to collect data and information” and added that if left unnoticed, the issue could have resulted in a catastrophic event.

The wheel issue is being blamed for an incident last week in which a train car slipped off the tracks on the Metro’s blue line near Arlington Cemetery.

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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 Washington Metropolitan Area Transit Authority 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.