The Sleeping Car Murders (1965)

Six people travel by overnight train from Marseilles to Paris. When they arrive, one of them, a young woman, is found dead in a sleeping berth. The police, led by Inspector Grazzi, investigate the other five passengers, suspecting that one among them was responsible. However, as the investigation is stepped up, the others start turning up dead. It's up to the remaining two to solve the case, lest they become the next victims.