"Mike" (played by Tyrone Power) works for the US State Department, delivering messages and packages. His latest assignment sends him to Trieste, where he’s supposed to meet someone on a train, pick up a crucial document, and bring it back home. Predictably, the Soviets are after the same thing, and when things don't go as planned, "Mike" gets caught up in something much more complicated than anything he’s dealt with previously. He now has to work under the orders of his new military contact, "Col. Cagle" (Stephen McNally), and a clever sergeant named "Guelvada" – Karl Malden gives an energetic performance as Guelvada – while trying to figure out if either of two women, "Janine" (Hildegard Knef) or "Joan" (Patricia Neal), is involved in this spy story, or if they aren’t. I thought the mystery wasn't very strong; things happen too conveniently in a wine bar, and after that, it seemed like the danger was lessened. Nevertheless, Henry Hathaway makes good use of the cast, creating an environment where everyone suspects each other, unsure who to trust and worried about being betrayed. Trains are often effective settings for thrillers – the constant motion, the dark compartments, and the sound of the rails all build up a feeling of suspense as we approach what’s supposed to be the big reveal. Maybe if you don't notice the obvious clue early on, you might enjoy this fast-moving thriller more.

Source on TMDB