The NTSB said early on that an overheated bearing on one of the railcars that was not caught in time by trackside sensors likely caused the crash. Investigative hearings have since highlighted other possible contributors including widespread rail job cuts and rushed inspections. Investigators also delved into why officials chose to deliberately blow open the vinyl chloride cars and burn that key ingredient for making PVC pipes.
A key point Tuesday will be the expected release of NTSB recommendations for safety improvements. Though NTSB recommendations aren't binding, it's possible Congress will be willing to enforce some of them because of the spotlight cast on rail safety by the crash.
More than a year ago, a bipartisan group of lawmakers led by Ohio's two senators proposed a package of reforms including requiring two-person crews and setting standards for the inspections and detectors that help prevent derailments. But that bill stalled in the Senate under resistance from Republicans and the railroads. GOP leaders in the House have said they didn't want to consider new rail safety regulations until after the final NTSB report was released.
With limited success, federal regulators also pushed for the railroads to make changes like signing onto an anonymous government hotline to report safety concerns. And the industry responded to the crash by promising to install more trackside detectors, review the way they are used and help first responders improve their handling of derailments with more training and better access to information about the cargo.
For his part, Norfolk Southern's CEO pledged to "make things right" in East Palestine with more than $100 million in aid to residents and the community. CEO Alan Shaw also hired a consultant from the nuclear power industry to recommend changes and tried to work with labor. Still, critics said Norfolk Southern was too often satisfied in the past with doing only the minimum required for safety and workers reported no big changes in day-to-day operations.
But after the derailment, all the major freight railroads pledged work to improve safety by adding hundreds more trackside sensors to help spot problems like overheating bearings and by re-evaluating how they analyze the data from those detectors. The Association of American Railroads trade group said the industry will review the NTSB report and look for additional ways to improve safety. But so far the industry's efforts haven't resulted in a significant boost in its safety record in the Federal Railroad Administration statistics.
Earlier this year, NTSB Chair Jennifer Homendy told Congress that the agency's investigation had determined that the controversial vent-and-burn operation that prompted evacuations and sent a huge plume of black smoke over the small town wasn't necessary. Experts from the company that made the vinyl chloride, OxyVinyls, testified at the NTSB hearings they were certain a feared chemical reaction that could have caused those tank cars to explode wasn't happening.
But Ohio's governor, first responders and the hazardous materials experts who made that decision have said the information they had that day made them believe an explosion was likely imminent, making the burn their best option despite the risks of unleashing cancer-causing dioxins in the area.
The chemical manufacturer has declined to comment publicly on the situation that is already the subject of lawsuits beyond what its experts testified to last spring.
Norfolk Southern announced last week that it will lead an industrywide examination of how to improve the way vent-and-burn decisions are made in future derailments. That was part of its settlement with the federal government.
The NTSB has also looked at the struggles of first responders who didn't immediately know exactly what was on the train after 38 cars jumped off the tracks, many spilling their contents and catching fire.
Federal officials finalized a new rule Monday that will require railroads to inform first responders about what is on a train immediately after a derailment. The industry says more than two million first responders now have immediate access to that information via an AskRail app that allows them to look up any train's cargo.
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Funk contributed to this story from Omaha, Nebraska, and Krisher from Detroit.