A wheel on the Norfolk Southern Corp. train that derailed earlier this month in Ohio was too hot in the moments before the accident, and a sensor failed to alert crew members to the problem in time, according to preliminary findings in an investigation by US authorities.

Surveillance video showed “what appeared to be a wheel bearing in the final stage of overheat failure,” the National Transportation Safety Board said Thursday in its report. 

A track-side detector located about 19 miles from the accident sensed that the bearing was more than 100 degrees hotter than the ambient temperature, but that was still too low to trigger an alarm, the NTSB said. As the temperature continued to climb, another sensor alerted the crew to the failure moments before the train went off the rails.

The preliminary report contains new details about the accident that has drawn national attention, but stops short of reaching conclusions about the underlying causes.

The freight train derailed in East Palestine, Ohio, on Feb. 3, releasing toxic chemicals in the area and prompting a brief evacuation of local residents. The disaster has put pressure on both Norfolk Southern and the Biden administration to ensure a proper cleanup effort is carried out. Some residents in the sparsely populated town have reported headaches, lingering odors and deaths of animals.

US Transportation Secretary Pete Buttigieg tweeted Thursday that he was visiting East Palestine to see the site and get an update on the investigation.

253 Degrees

So-called hot bearing detectors are sensors placed periodically on railroad beds to help detect the kind of failure that occurred on the Ohio train by looking for overheated wheel assemblies.

The bearing that failed was was 38 degrees above the ambient temperature when it crossed a sensor about 29 miles before the derailment, the NTSB said Thursday. It had risen to 103 above normal 10 miles later, and spiked to 253 degrees above ambient temperature at about the time of the failure.

Only the third sensor prompted an alarm because Norfolk Southern’s system doesn’t require inspections on overheated bearings until they reach 170 degrees above ambient temperature, or a difference of 115 degrees between bearings on the same axle, the NTSB said. The train crew applied the brakes after getting an alert, but it was too late to prevent the accident.

The NTSB said its investigation is still ongoing. The agency will focus in part on the wheelset and bearing; tank car design and derailment damage; and a review of the accident response in the future.

The Environmental Protection Agency this week called on Norfolk Southern to conduct cleanup operations in accordance with an approved plan and pay for remediation costs. The company has pledged to pay for the work and reimburse residents for the disruption.

At a news conference, the governors of Ohio and Pennsylvania raised the prospect of criminal charges, saying the state attorneys general were reviewing the matter.

Norfolk Southern shares were little changed at 10:26 a.m. in New York.