Updated February 23, 2023 at 2:55 PM ET

The operators of a Norfolk Southern train carrying hazardous chemicals near East Palestine, Ohio, earlier this month tried to stop the train after a wheel bearing overheated to a dangerous degree, the National Transportation Safety Board said.

But by the time an alarm rang to alert the crew of the danger, the bearing was already so compromised that it failed as the train was still coming to a stop, officials said — a revelation that raised questions about whether Norfolk Southern's safety sensors and procedures were sufficient to prevent a catastrophic failure.

"Roller bearings fail. But it's absolutely critical for problems to be identified and addressed early so these aren't run until failure," said NTSB Chair Jennifer Homendy at a Thursday press conference.

A preliminary report released by the NTSB Thursday stopped short of declaring a conclusive cause of the derailment. But the report, along with additional details shared by officials, together offered the most detailed explanation yet of what may have sent the train off the tracks.

Norfolk Southern train 32N, a 149-car, 9,000-foot-long train traveling east along the railroad's Fort Wayne Line across Ohio, derailed near East Palestine just before 9 p.m. on Friday, Feb. 3.

As the train approached East Palestine, a wheel bearing on its 23rd railcar rapidly overheated, its temperature soaring to more than 250 degrees Fahrenheit above the ambient outdoor temperature of 10 degrees.

The overheating triggered an alarm, which caused the train's engineer to immediately apply the brakes to bring the train to the stop. An automatic emergency braking system also came into effect.

But when the three-person crew exited the train to inspect the bearing, they saw fire and smoke, an indicator of a possible derailment, the report said. Ultimately, 38 cars derailed, 11 of which carried toxic chemicals.

Trackside detectors saw temperatures spiking in a wheel bearing

Cited in the NTSB's report was data collected by Norfolk Southern defect detectors, devices built along railroad lines with sensors that detect and report problems with signals and axles as trains pass by.

Three such sensors, also called hot-box detectors, were located along 30 miles of track near East Palestine. They recorded increasing temperatures in the suspect wheel bearing, investigators said.

The first detector recorded a temperature of 38 degrees Fahrenheit above the ambient temperature, the report said.

By the time the train reached the second detector 11 miles later, the bearing had reached 103 degrees above the ambient temperature.

The third and final detector, located 19 miles later just east of East Palestine, recorded such a high temperature — 253 degrees Fahrenheit above ambient temperature — that the train's crew was alerted to stop the train and inspect the bearing in accordance with Norfolk Southern safety guidelines, the NTSB report said.

Only the third reading was high enough to trigger an alarm, according to the report.

On hearing the alarm, the engineer "responded immediately" to begin bringing the train to a stop, Homendy said, but as the train decelerated, the wheel bearing failed.

Norfolk Southern safety guidelines do not require train operators to take action until wheel bearings reach 170 degrees Fahrenheit above ambient temperature, the report said. Once a critical level of 200 degrees Fahrenheit is reached, Norfolk Southern requires its train operators to stop a train immediately and remove the affected car from the train.

More frequent sensors may have helped

But with 19 miles between detectors, the wheel bearing heated beyond the train operators' ability to stop the train safely, the NTSB said.

"Had there been a detector earlier, that derailment may not have occurred. But that's something we have to look at," she said.

Investigators are still examining the axle and wheel bearing, along with parts of the tank cars that carried toxic chemicals. The NTSB will also review Norfolk Southern's safety equipment and procedures, officials said.

Once the crew had stopped the train, operators saw fire and smoke and alerted authorities of a possible derailment. Responders arrived soon after.

The train's movements at the time of the derailment appeared to follow safety regulations, investigators said. It was traveling at 47 miles per hour, just under the maximum speed limit of 50 miles per hour. Positive train control, an automated safety system, was enabled and operational.

"We have no evidence that the crew did anything wrong," Homendy said.

The incident has prompted the Department of Transportation to consider new safety regulations, said Secretary Pete Buttigieg on Thursday during a visit to East Palestine.

The spacing of hot-box detectors and the temperatures at which they trigger alarms are not currently regulated by federal law, officials say.

"There are differences from railroad to railroad, from company to company, in terms of how they're used. I think that's another example of something that needs to be looked at to try to prevent things like this from happening again," Buttigieg said.

More than 100,000 gallons of vinyl chloride burned

There were no reported fatalities or injuries caused directly by the derailment, and responders were able to mitigate the fire within two days. But rising temperatures in one tank car carrying vinyl chloride — a toxic chemical that grows especially volatile at high temperatures — prompted concerns of an uncontrolled explosion. To prevent that, responders evacuated residents of the immediate area and performed a "release and burn" of five tank cars' worth of the chemical, some 115,580 gallons.

The plastic placards marking cars with hazardous materials have also been identified as an area of concern by both Buttigieg and NTSB officials. The fire following the derailment melted the plastic placards, making it difficult for first responders to discern which trains contained dangerous chemicals.

The evacuation order was lifted on Feb. 8, five days after the derailment. Since then, residents have reported widespread concern about the safety of East Palestine's air and water. Some say they have experienced headaches or rashes. The Ohio Department of Natural Resources reported that some 3,500 fish had died following the derailment in nearby waterways.

The Environmental Protection Agency has ordered Norfolk Southern to conduct a cleanup of the chemicals from the area's soil and water.

NPR's David Schaper contributed reporting. contributed to this story

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