One Night... A Train (French: Un soir, un train) is a 1968 Belgian-French drama film directed by André Delvaux, starring Yves Montand and Anouk Aimée. It tells the story of Mathias, a professor of linguistics at a university where the students have lively discussions about Flemish nationalism and morality. During a train ride, the French-speaking woman who Mathias lives with disappears and he goes looking for her in an unknown city. The film is based on the novel De trein der traagheid by Johan Daisne.