Terror by Night (1946)

Holmes is hired by Roland Carstairs to prevent the theft of the Star of Rhodesia, a 423 carat diamond owned by Carstairs' mother, Lady Margaret. Suspecting the diamond will be stolen on a train trip from London to Edinburgh in Scotland, Holmes deftly switches diamonds with Lady Margaret while in her compartment admiring the jewel. Soon after, Roland is murdered and the fake diamond is stolen. Red herrings abound as Holmes, aided by Dr. Watson and Inspector Lestrade, discover the murderer's hiding place and deduce that long-time nemesis Professor Moriarty's henchman, Colonel Sebastian Moran, is somehow involved in the crime.