"The Monomaniac (La bête humaine)" by Émile Zola
Lantier, the "human beast" of the title, has a hereditary madness and has several times in his life wanted to murder women. At the beginning of the story he is an engine driver, in control of his engine, "La Lison." His relationship with "La Lison" is almost sexual and provides some degree of control over his mania. As a result of a chance remark, Roubaud suspects that Séverine has had an affair some years earlier, with Grandmorin, one of the directors of the railway company, who had acted as her patron and who had helped Roubaud get his job. He forces a confession out of her and makes her write a letter to Grandmorin, telling him to take a particular train that evening, the same train Roubaud and Séverine are taking back to Le Havre.