During a layover in Paris, Roubaud comes to suspect that Séverine had an affair years ago with Grandmorin, her patron when she was an orphaned teenager. Roubaud forces a confession from her about being seduced by Grandmorin. In a jealous rage, Roubaud makes her write a note to the judge, asking for a rendezvous on that evening's express train from Paris. It is the same train Roubaud and Séverine are taking back to Le Havre. Lantier is afflicted with hereditary madness that causes his sexual desires to turn into fits of violence against women. His relationship to his engine, which he affectionately dubs "La Lison", provides him some relief. While staying in Paris to have "Lison" repaired, he visits his Aunt Phasie. When he encounters Flore, his passions are inflamed and trigger his homicidal mania. He feels on the verge of stabbing her, but restrains himself and runs away. Standing near the railway line, Lantier observes the express from Paris go by. Through a passing window, he glimpses a man holding a knife, bent over another man. Lantier later finds Grandmorin's body beside the track with his throat cut. Roubaud and Séverine are suspects since they were on the train with Grandmorin and will inherit some of his property. Lantier sees Roubaud at the police station and thinks he resembles the murderer he glimpsed, but when questioned says he cannot be sure. The magistrate, pursuing a false lead against Cabuche, dismisses Roubaud and Séverine. Even though they are cleared, the marriage of Roubaud and Séverine becomes strained. The cash and watch they stole from Grandmorin are hidden under a floorboard in their apartment. As "insurance" against a change in Lantier's testimony, Roubaud cultivates a friendship with the engine driver and encourages his wife to do so. Séverine and Lantier begin an affair, at first clandestinely but then more openly until they are caught by Roubaud. Despite his previous jealousy, Roubaud seems unmoved; he spends less time at home and turns to gambling. One night, Séverine admits to Lantier that Roubaud murdered Grandmorin, and she assisted. Lantier feels the return of his own urge to kill. The next morning, he leaves the apartment to kill the first woman he meets. After choosing a victim, he is seen by someone he knows and abandons the idea. He is relieved and hopes his murderous inclinations will plague him no longer. Séverine discovers Roubaud has taken all the hidden cash stolen from Grandmorin to pay off gambling debts. She suggests to Lantier that they sell the property she and Roubaud inherited from Grandmorin, and use it to escape to New York where Lantier has heard of a business opportunity. Roubaud is the only obstacle to this new life, and they decide to kill him. They approach him late at night when he is doing his rounds as a watchman at the station, figuring the murder will be attributed to robbers. At the last moment, however, Lantier loses his nerve. Meanwhile, the lovesick Flore sees Lantier regularly pass her house driving the train. She notices that Séverine always rides on his Friday morning express to Paris. Flores realizes the two are having an affair and grows fiercely jealous, wishing to kill them both. She hatches a plot to derail Lantier's train while Séverine is on board. Flore seizes her chance when Cabuche leaves his wagon and horses unattended near the railway line. She leads the horses onto the tracks seconds before "Lison" arrives. In the resulting crash, numerous people die, but Séverine is unhurt and Lantier is moderately injured. Wracked by guilt, Flore commits suicide by walking in front of a train. Séverine nurses Lantier back to health. She convinces him they must kill Roubaud, and they concoct a plan. But Lantier's atavistic madness returns, and when Séverine tries to make love with him shortly before Roubaud is due to arrive, he murders her. Cabuche finds Séverine's body and is accused of killing her at Roubaud's behest. Both men are put on trial for the crime, and for the murder of Grandmorin. They are convicted and sentenced to life imprisonment. Lantier begins driving again, but his newly assigned engine is just a number to him. He initiates an affair with Pecqueux's girlfriend. Lantier drives a train carrying troops towards the front at the outbreak of the
Franco-Prussian War. The resentment between Lantier and Pecqueux breaks out into a fight as the train is travelling at full steam. Both fall to their deaths, and the train loaded with soldiers hurtles driverless through the night. ==Themes and criticism==