Sherlock Holmes (Basil Rathbone) and Dr Watson (Nigel Bruce) are hired by Roland (Geoffrey Steele) to protect the valuable diamond owned by his mother Lady Margaret (Mary Forbes) as they travel back up to Edinburgh on the train. However before very long Roland is found dead and the diamond is missing! The murderer and thief must be still on the train.
The investigation begins of the other passengers on the carriage, several of whom are suspicious though there are red herrings a-plenty even a mysterious coffin. Sherlock Holmes deduces that Moriarty's henchman Moran is involved...
Although low-budget (nearly all the film takes place on one set) this is an excellent little mystery. Though it is somewhat marred by Watson's attempts at humour and some dodgy accents.