When planning the train robbery they calculate a time frame of 20 minutes. When the robbery actually takes place, the sequence is exactly 20 minutes long.
Director Jean-Pierre Melville built a wooden cabin on the set during shooting of the film and stayed inside it most of the time, refusing to talk even to technicians or actors.
While Alain Delon's character, Commissaire Coleman, examines a crime scene, we see a brief shot of a wall on which are inscribed several names, including "Jef Costello". Jeff Costello was the character Delon played in one of his previous collaborations with Jean-Pierre Melville, Le Samouraï (1967).
During filming, director Jean-Pierre Melville ordered Florence Moncorgé-Gabin, who worked on this film as the script supervisor, to wear a wig on set, because he did not like her hair color. Moncorgé-Gabin is the daughter of legendary actor Jean Gabin.