Thomas & Friends Trivia or Fun Facts Game

It's a real life British slang term. It has no relation to The Railway Series nor Thomas & Friends (which are already gruesome enough), but I've seen it mentioned enough times to remember it. For example, I recall it being used by British rail enthusiasts who build LEGO trains, and on the TTTE Wikia. It's so common that a website named after it has THREE registered domains. I never knew about "Plum and Spilt Milk" though.