That would sound like a hotbox on one of the tank cars. If so there will be a post mortum one was the packing insufficient or was the journal box not inspected correctly or on time. It could take quite a while to fix blame.
Article I read, which included a map, reported that the train passed on hot box after the video was captured, then another hot box twenty miles later, which was (IIRC) about the miles before derailment.
First box inoperable, or warning ignored?
It’ll come out eventually.