Terror by Night (1946)

Director: Roy William Neill
Cast: Alan Mowbray, Basil Rathbone, Dennis Hoey, Frederick Worlock, Mary Forbes, Nigel Bruce, Renee Godfrey, Skelton Knaggs

Synopsis:

Holmes and Watson board a passenger train bound from London to Edinburgh, to guard the Star of Rhodesia, an enormous diamond worth a fortune belonging to an elderly woman of wealth; but within the first hour of the trip, the woman’s son is murdered and the diamond stolen and any of the passengers in their car could be the killer thief.