SOLVED. A book where a group of people steal a diamond and escape in a steam locomotive. The railway leads them to an old factory building where there is a mental hospital [s]

[Vera]

I also remember that one of the mental hospital employees call the police saying that the thieves came by an old locomotive and the police laugh at them.

[Lobstergirl, au gratin]

Fiction or nonfiction?
When did you read it?
Or, when was it published?
Author female or male?
When and where is it set? Country/region/city etc.
Remember the cover?
What's the tone - comedic, jaunty, quirky, suspenseful, dramatic?
What's the genre? Adventure, drama, comedy, etc.?

[Peter]

I think this is The Hot Rock by Donald Westlake. It’s the first of his excellent series about brilliant but hapless thief John Dortmunder. The plot has Dortmunder and his crew forced to steal the same jewel at least six times. They use the locomotive to break into an asylum where a man who knows the jewel’s location is hiding. He had himself committed because he thought it’d keep him safe, but not so much.

[Vera]

Peter wrote: "I think this is The Hot Rock by Donald Westlake. It’s the first of his excellent series about brilliant but hapless thief John Dortmunder. The plot has Dortmunder and his crew forced ..."

Thank you very much! It's the very book I've been looking for