[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.

[jerseygirl]

That's exactly what my son said. Still, if it were a toxic gas, would the inspectors be walking through the train and looking under it without gear?

Another thought (not knowing anything about trains)- apparently this train runs on diesel until you reach Philly, there, they switch to an electric engine.

Could the diesel have been on fire maybe? The gentleman in the ticket booth said "mechanical problems".