Burning hazardous material is almost always a bad idea if it is uncontrolled. In controlled environments such as a flare or incinerator the risks are minimal. If they implemented the proper controls (e.g. air monitoring, evacuation, proper PPE) it can be a useful tool to prevent a worse outcome (e.g. a chemical explosion). I don't know the exact circumstances under which they had to make that decision but it should have been their last resort. If it comes out that there was a safer way and they just did it to save money or avoid rail line closure times, then that's pretty shitty and it potentially exposed the workers and nearby residents to hazardous chemicals for profit reasons.