The N.C. Department of Transportation on Saturday ordered the Charlotte Area Transit System to pull all of its older light rail vehicles from service, according to a letter the city sent to the Metropolitan Transit Commission.

The state acted after a light-rail driver detected a problem with one wheel: a “flat spot.”

In late December, a Lynx train was pulled from service because of an apparent flat spot on wheel. It’s a small imperfection that’s measured in millimeters, and it can make a clicking sound.

The car was repaired on Dec. 30 and put back into service.

On Friday, a driver operating the same train reported the same sound. The vehicle was pulled from service again.

While being repaired, a CATS machinist “noticed an abnormal sound coming from the center truck,” according to the DOT.

The light rail vehicle was inspected again. That showed there was “abnormal movement” coming from one wheel of the train.

CATS contacted the DOT, which oversees safety at the transit agency, on Saturday about what had happened. The state ordered CATS to remove older Lynx trains on Sunday.

In an email to the Metropolitan Transit Commission, one of the governing bodies for CATS, the transit system said it doesn’t believe that this wheel problem is related to a previous problem with ball bearings in wheel axles, which caused a May 2022 derailment.

CATS also said it doesn’t believe the “flat spot” wheel problem is a safety issue.

The agency also said it can maintain the current Lynx schedule, even without roughly half of its vehicles. CATS said it may have to adjust schedules in the future, depending on its maintenance schedule from the manufacturer, Siemens.

The DOT’s order impacts the 100 and 200 series vehicles from the Lynx Blue Line. The 300 series vehicles — which came on line when the Lynx extension opened in 2018 — will be used for now.

CATS said it has inspected all of its light-rail vehicles and hasn’t found any similar problems.

In the email to the MTC, interim CATS chief executive Brent Cagle praised the transit agency’s response. “This is an exemplary demonstration of a system that truly works. Staff proactively collaborated across various divisions to address the mechanical concerns, ensuring these were promptly reported to NCDOT. This is a testament to our collective effort to uphold our commitment to safety for our employees and passengers,” CATS wrote.

The agency’s response to the recent problem is a marked departure from the safety issues that first emerged almost two years ago. John Lewis was the CATS chief executive then.

After the May 2022 derailment — which didn’t cause any injuries — the DOT blasted CATS for its response to the accident. It criticized CATS for not doing the required maintenance on its older vehicles and ordered a system-wide speed limit of 35 mph for all trains.

And CATS staff didn’t inform Cagle about the problem when he took over as the transit system’s leader, instead keeping him in the dark and contributing to an embarrassing series of disclosures about safety problems including an understaffed rail control center and missed inspections on bridges and other critical infrastructure.