The gold is in the firebox of the engine! What if somebody decided to build up a head of steam and struck a match to it?
8 seconds sound clip from the The Train Robbers movie soundboard.
You can hear this line at 00:48:15 in the Blu-ray version of the movie.
Quote context
[...]
- Well, you heard what the woman said.
- I'm not talking about the train.
- I'm talking about the half of million.
- You wouldn't catch me hiding that much gold in the boiler of a six-wheeler!
- I'll believe that when I see it.
- Wonder whatever happened to that old boy.
[...]