Sunday Night Movie: Sleeping Car to Trieste (1948) – Jean Kent, Albert Lieven, David Tomlinson, Derrick De Marney, Rona Anderson,

Wikipedia

The setting is almost entirely on a train travelling between Paris and Trieste after World War II. Two rather mysterious people, Zurta (Albert Lieven) and Valya (Jean Kent), are at ease in sophisticated society. Zurta steals a diary from the safe of an embassy in Paris while they are guests at a reception there, killing a servant who walks in on the robbery. Poole, an accomplice, is passed the diary, but he double-crosses them and attempts to escape with it on the Orient Express. Just in time, Valya and Zurta board the train.

They start looking for Poole, who seeks to conceal himself and the diary. Other travellers become involved, including a US Army sergeant with an eye for the ladies, an adulterous couple, an idiot stockbroker, a wealthy, autocratic writer and his brow-beaten secretary, an ornithologist, and a French police inspector. Staff and other passengers provide light-hearted scenes. The diary passes through the hands of several people while the police investigate a mysterious death.