Kawalerowicz does an exceptional job of keeping such a simple thriller so unbearably tense for the duration of ninety minutes. He reveals nothing about Jerzy; why he's behaving as he is, why he has no ticket, or will pay double to get a private car - to point the finger of suspicion at him. Marta acts quite similarly though we suspect this is the fallout of the collapse of her relationship with Staszek (and there are ominious razorblade cuts on her wrists) - plus, we never suspect her as we know the murderer is male. Both seem equally lonely and abandoned, though they never bond. In fact they want to stay out of each other's way as much as possible, though we're never told why.
Beginning with a trademark bird's eye view of the passengers running for the train (used to such great effect in Mother Joan of the Angels), Kawalerowicz creates a claustrophobic environment for his principle characters to inhabit, which is increased greatly when suspicion falls upon the pair and they start a collective mentality of persecution. One thinks of the wicked whispers of Clouzout's 'Le Corbeau' (1943) when a gang mentality can be so cruel and vindictive without much justification. As a thriller, it belongs in the Hitchcockian school (who had his own partially train-based thriller that year with North by Northwest) - the tension is unbearable and well handled at all times, and Kawalerowicz pursues forever relevant themes to his credit.