Something to do with gas pressure?
The observant will notice that it's not sitting on the rails. My vote is for some kind of safety demonstration.I think it was steam cleaned and then allowed to cool with the valves shut, so yes like a milk jug.
http://blogs.howstuffworks.com/2008/04/22/how-tank-car-implosions-work/No, but the question was how it happened. It's still an awesome display of the effects of pressure.