[jerseygirl]

I was guessing it was an electrical fire of some kind, but my son pointed out that the smoke was billowing out and settling, not rising. Would smoke from an electrical fire settle?
The people inspecting the train weren't wearing any kind of protective gear or anything.

[Myrddin]

When I see a gas behaving as "heavier than air", I'm immediately suspicious of a toxic gas. Smoke from a fire tends to be hot and rises.