A jury is made up of twelve citizens who walk into a courtroom and decide whether a defendant is guilty or innocent. Murder on the Orient Express plays with the same idea. There are twelve people on the train who act as jury and executioner of a criminal. Justice is served in a way that it wasn't guaranteed in the courts. Justice is often juxtaposed with revenge in this novel, forcing us to think about the ways in which the two are similar and different.
When it comes to justice, an eye should be given for an eye.
It was wrong of Poirot, at the beginning of the novel, to refuse to help Ratchett solely on the basis of Ratchett's appearance.
Dr. Constantine and M. Bouc made the right decision when they decided not to tell the truth to the police.