The NTSB confirmed the crash was caused by an overheating bearing on one of the rail cars, and they detailed why officials were wrong to blow open five tank cars of vinyl chloride and burn the contents.

Here are some of the agency’s key findings from Tuesday’s hearing:

Trackside detectors

The detectors railroads use along their tracks to help spot overheating bearings, flat wheels and dangling equipment were a key focus of the NTSB investigation.

The Norfolk Southern train that derailed in East Palestine passed three so called “hot box detectors” just before the crash, but the train’s overheating bearing wasn’t caught in time even though surveillance footage showed a fire underneath the rail car as it passed through Salem, Ohio.

The detectors did notice the temperature increasing, but didn’t signal an alarm soon enough. NTSB investigators said the detector in Salem didn’t get an accurate temperature reading even though it showed the bearing was 103 degrees hotter than the outside temperature. That’s partly because it can take a while for the heat from a burning bearing to reach the outside of the axle where it can be measured.

The NTSB said more research and rules are needed on detectors because there are no federal standards for them. Major railroads developed the devices on their own without guidance on where they should be placed or when they should trigger an alarm. Industry research has shown that having hot box detectors every 15 miles is ideal, but investigators think more study is needed.

After East Palestine, the six biggest railroads promised to install hot box detectors an average of 15 miles apart and adopted a uniform standard that trains should be stopped anytime a bearing registers more than 170 degrees above the ambient temperature.

But neither one of those measures seem like they would have changed anything in this derailment. Even though the Salem and East Palestine detectors are 20 miles apart, the previous one was just 10 miles away so they already averaged 15 miles of distance between them. Norfolk Southern was already using 170 degrees as its threshold.

Troubled Tank Cars

The East Palestine derailment highlighted longstanding concerns about certain tank cars known as the DOT-111. Three of the hazardous materials cars that derailed, ruptured and caught fire that night were that model. NTSB Chairwoman Jennifer Homendy said if those cars hadn’t ruptured, there may never have been the massive dayslong fire that prompted officials to needlessly blow open five vinyl chloride tank cars and burn their contents three days after the derailment. Authorities did so because they were worried the cars would explode.

The DOT-111 tank cars are made with a steel shell less than half an inch thick that time and again has proven much more likely to rupture than newer cars made with thicker steel.

The same tank cars figured in the worst rail disaster in modern history when 47 people were killed after a crude oil train derailed in the small Canadian town of Lac Megantic in 2013. DOT-111 tank cars were also involved in a number of disastrous crude oil and ethanol derailments in the early 2000s when railroads routinely hauled entire trains of those flammable commodities.

Regulators issued a rule in 2015 that was designed to get all the DOT-111s carrying flammable liquids replaced or upgraded by 2025, but Congress delayed the deadline until 2029. The tank car owners — which are generally the chemical companies, other shippers and leasing companies rather than the railroads — have long resisted a more aggressive upgrade schedule because of the roughly $135,000 price tag of a stronger DOT-117 tank car.

About 25,000 DOT-111 tank cars are still in use, according to the Association of American Railroads. That’s a relatively small part of the North American tank car fleet of about 450,000. Many of them carry much more harmless cargo such as corn syrup.

Despite concerns the NTSB has been raising at least since 1991, current rules will still allow DOT-111 tank cars to haul some hazardous materials, such as combustible liquids like diesel, even after the 2029 deadline. They just won’t be able to be used for things classified as flammable liquids like the butyl acrylate that spilled in East Palestine.

Delayed Reforms

Rail labor groups and safety advocates hope that now that the NTSB has had its say, Congress will finally act on reforms that stalled months after East Palestine. Republican leaders said they wanted to see that agency’s final report before they considered imposing new rules.

In the Senate, proponents for the rail safety bill have continued to express optimism that the legislation could get a vote on the floor. But so far opposition from Republicans and the railroads has kept the bill from moving forward. Similar legislation has failed to gain any momentum in the Republican-controlled House.

It’s expected that Republicans will propose a much narrower rail safety that likely won’t include all the inspection standards and two-person crew requirements that are in the Senate bill now. With attention turning toward the November election, congressional leaders have little time left to complete drawn-out negotiations that significant rail safety legislation would likely require.

Crisis of Confidence

Homendy refused to bow to pressure from Norfolk Southern to declare that the railroad didn’t press to vent and burn the vinyl chloride cars because it wanted to get the trains moving again quickly. The railroad has long insisted that it was worried about safety — not its delivery schedule or bottom line — when it recommended that last resort.

Railroad safety experts said it’s true that the vent and burn strategy was the quickest way to reopen the tracks. Waiting for the fire to go out and unloading the damaged tank cars with trucks might have taken weeks.

“When you’ve got 35 or 45 trains sitting and waiting to get through one area, no way to get around it,” said Randy Fannon, who leads the Brotherhood of Locomotive Engineers and Trainmen’s safety task force and helped with the NTSB investigation.

“The railroad wants to get their main line back open as fast as possible,” he said.

The NTSB’s findings and Homendy’s critique prompted new calls for accountability. Norfolk Southern already settled with the federal government and agreed to a $600 million class action settlement with residents. State investigations in Ohio and Pennsylvania and individual lawsuits appear to be the only remaining potential consequences.

Ohio Attorney General Dave Yost said: “The NTSB findings answered some questions but raised additional questions. Our lawsuit is ongoing.”