I remember doing this experiment in grade school (with a can--not a tank car!).
This was back in the days when it wasn't neccessary to have disclaimers in the textbook about not touching hot things and don't drink the chemicals.
I guess science education isn't what is used to be.
Here's what happens: the steam displaces air in the container. Then it's sealed and allowed to cool. As it cools the steam condenses into water creating a partial vacuum in the container. The atmopheric pressure of 14. 7 PSI outside the container does the rest.