Terror By Night
From IMDb: Holmes is hired by Roland Carstairs to prevent the theft of the Star of Rhodesia, an enormous diamond owned by Carstairs' mother, Lady Margaret. Believing the diamond will be stolen on a train trip from London to Edinburgh, Holmes deftly switches diamonds with Lady Margaret while in her compartment. 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 foe Moriarty's henchman Colonel Sebastian Moran is somehow involved in the crime.