A great deal has been written about the train derailment in East Palestine, Ohio. The preliminary NTSB report is one of the few solidly factual responses. One important question has been: what happens when vinyl chloride burns? Theoretically, in the presence of enough oxygen, you get this:
More realistically, based on the above reaction and this paper, you also get carbon monoxide, black soot (carbon), and traces of phosgene:
Why was it burned? Because vinyl chloride, if it gets too warm, can spontaneously polymerise into PVC plastic. That reaction is heat-producing and can lead to an explosion. Since the fire following the derailment had heated the rail cars containing vinyl chloride to dangerous levels, authorities believed an explosion was imminent. A controlled burn was probably a rational decision at the time.
Hydrogen chloride (HCl) in the smoke cloud was probably the immediate threat resulting from the controlled burn (and the likely cause of dead birds), although the HCl would soon have been safely diluted by rain. Unburned vinyl chloride in the subsoil is probably the longer-term threat, and (I understand) the focus of cleanup efforts.
Why did the train derail? This (somewhat fuzzy) map and chart is my best guess at a timeline. As the train travelled east, hotbox detectors (HBDs) noted increasing wheel bearing temperatures on car 23. Some media reports suggest flames of burning axle grease were seen in Columbiana, Ohio. The HBD at East Palestine noted a temperature of 253°F above ambient, higher than the railway company’s critical threshold. The crew immediately began to further slow the already slowing train, at which point the faulty wheel bearing on car 23 failed catastrophically, triggering the derailment and fire:
The map at database.defectdetector.net suggests that there used to be hotbox detector near Columbiana, Ohio (at MP 60.8), and this would presumably have caught the fault in time to avoid a derailment. One wonders what became of that HBD. It’s a pity that we need to wait 18 months for the NTSB’s final report.
Meanwhile, the EPA has put all their air, soil, and water testing results online.