Terror by Night (1946)

A wealthy young aristocrat employs "Holmes" to escort himself and his mother on a train from London to Edinburgh alongside the legendary Star of Rhodesia - a diamond the size of a duck egg. Together with trusty sidekick "Dr. Watson" and "Insp. Lestrade" who is ostensibly off fishing for salmon (it isn't quite the season) they embark on a singularly dangerous train journey. It's got plenty of dud trails for our duo, even some appropriated tea pots and a coffin - before we finally identify that the arch criminal "Col. Moran" is also on the train and he will stop at nothing to procure the ice. The pace rattles along as quickly as the train, and builds nicely to quite a quirky conclusion with a few near misses along the way. Great way to spend a hour!