Burning of the chemicals hauled on the train cars produced phosgene, which has been used as a weapon.

Directly following the crash, crews began a controlled release of the chemicals, fearing an explosion. At the time of the release, officials warned that the burn would send phosgene and hydrogen chloride into the air. Phosgene being a toxic gas that can cause vomiting and breathing trouble that was used as a weapon in World War I.

Source

The article cited, however, is misleading. The mainstream news narrative makes it sound like the fire began during or on the same day as the derailment. The fire was started by the train company three days after the train derailed, but the article did not give a specific time, but said "directly following the crash".