I believe this is a remake of the film Rome Express.
In the beginning of this film, we see Zerta (Albert Lieven), a foreign agent, steal something from the Paris Embassy. He then throws it out the window to the waiting Karl. Valya (Jean Kent) is also at the embassy and an accomplice. They are to meet Karl the next day, but he stands them up. Realizing he is going to sell whatever it is for more money, they take off attempting to find him.
They find out that he is on a train headed for Zagreb. That's when the fun begins. Karl is upset to find that he does not have his own berth, which he has to have in order to a) stay hidden; and b) hide what he stole.
There are a bunch of characters involved - a couple secretly committing adultery, a friend of the man part of the couple (David Tomlinson) who wants to play cards and drink; a wealthy snobbish man (Finlay Currie) and his harried assistant; a bird watcher who won't shut up; two French girls who take advantage of an American soldier to avoid customs; the chef, who has to listen to an amateur cook that won't shut up.
Eventually we learn that the object is a diary that, if published, could start another war. Karl finally manages to get a berth alone, only to be moved from it after he's hidden the diary. The adulterous man would like his girlfriend to come to his berth, but he's stuck with someone else unexpectedly in his berth.
Sooner or later, they all play their part in retrieval of the diary.
Entertaining post-war film, well-directed.
