The book’s most significant theme is that of justice versus retribution. By the end of the book, the reader will have to ask themselves whether the need for justice outweighs the need for retribution. Poirot will be forced to address the question more concretely when he must choose between the two theories of the murder he’s developed—a correct theory that prioritizes justice (by imprisoning 12 murderers) versus an incorrect theory that prioritizes retribution (by letting those same 12 murderers walk free, all in the name of justice).

Colonel Arbuthnot first broaches the concept of retribution. When he “learns” Mr. Ratchett’s true identity is Cassetti, he says, “The swine deserved what he got. Though I would have preferred to have seen him properly hanged—or electrocuted, I suppose, over there” (133). Poirot makes note of the Colonel’s preference for retribution, in that the man’s response illustrates his own ideas of how justice could have been delivered.

As the narrative continues, it becomes clear that “justice” failed Daisy Armstrong, a little girl who Mr. Ratchett (Cassetti) kidnapped and murdered. The man who was slain in the train’s compartment becomes less a victim as his crimes are revealed and the continued losses included the girl’s mother, unborn sibling, father, and nursemaid.