https://en.wikipedia.org/wiki/Rome_Express

Based on a story by Clifford Grey, with a screenplay by Sidney Gilliat, the film is a tale about a European express train to Rome carrying diverse characters, including thieves, adulterers, blackmail victims, and an American film star.^[4] The film won the American National Board of Review award for Best Foreign Film.^[4][5] Rome Express was remade as Halfway to Shanghai” 1941 and  Sleeping Car to Trieste (1948).

The film is centred almost entirely on the Compagnie Internationale des Wagons-Litstrain the Rome Express, travelling between Paris and Rome.

Just as the train is departing from a station in Paris, Zurta and his colleague Tony just manage to board the train. They have found out that someone they want to see is on board. Another passenger, McBain, a wealthy businessman travelling with his brow-beaten secretary/valet, Mills, learns that a painting by Van Dyck, which he had previously tried to buy and which had later been stolen, has not been recovered and says he would do anything to obtain it. Also on the train are an adulterous couple, an annoyingly sociable Englishman, a French police inspector, and an American film star who is tiring of her fame, accompanied by her manager/publicist.

It transpires that the stolen painting is in the possession of a man, Poole, who conspicuously keeps his briefcase close to him at all times…..

And so on.