The passengers crowd into the restaurant car and sit around the tables. Greta Van Ohlsson is still weeping. Poirot announces that there are two possible solutions to the crime, he will present both and then Dr. Constantine and M. Bouc will decide which one is correct.
Poirot reveals the first solution:
Mr. Hardman's evidence shows that no one passed in or out of the Stamboul- Calais coach. The enemy described to Hardman by Ratchett joined the coach at Vincovci through the door left open by Arbuthnot and McQueen. Wearing a Wagon Lit Uniform, the enemy went into Ratchett's compartment and murdered him. Ratchett's watch was found stopped at 1:15 because Ratchett forgot to set it back at Tzaribrod. The murder was committed at 12:15 a.m. The voice heard at 12:47 was a third person in the compartment.
Poirot expounds the second possible solution:
The first day on the train, M. Bouc said something curious to Poirot; "the company assembled was interesting because it was so varied—representing...all classes and nationalities. Poirot realized that only such an assembly would be possible in America, which led him to the guessing scheme.
McQueen's second interview tipped off Poirot. When Poirot told him that a note with the name Armstrong had been found he starts, "But surely—" and did not complete his sentence. Poirot felt he had started to say, "But surely that was burnt." This convinced Poirot that McQueen was somehow involved with the murder.
Masterman's insistence that Ratchett always took a sleeping drought was suspicious. Ratchett wouldn't have taken a sleeping drought when he was convinced someone was trying to murder him.
Hardman's evidence that no one passed in or out of the coach confirmed that the murderer was in the Stamboul-Calais coach. The conversation Poirot overhears between Miss Debenham and Colonel Arbuthnot reveals that they were on terms of intimacy, not strangers just meeting on the train. Arbuthnot called her "Mary." It is highly unusual for an Englishman to call a woman by her first name unless he knows her very well.
Mrs. Hubbard's story that she had Greta Ohlsson check to see whether the communicating door was bolted was obviously false—she could see it herself, the bolt is a foot above the door handle.
The cry that Poirot heard at 12:47 was obviously not Ratchett, because Ratchett was drugged and there were no signs of struggle. The voice that Poirot heard was also not Ratchett, he could speak no French.
Poirot believes that the scene at 12:47 was carefully planned and staged. Ratchett was actually murdered close to two in the morning.
The extraordinary difficulty of convicting any one person in the crime and the amount of people traveling on the train with some connection to the Armstrong case led Poirot to one solution—they were all in it. The Armstrongs formed a self-appointed jury of twelve came together and stabbed Ratchett twelve times. Ratchett escaped justice in the US, but the Armstrong family carried out their own form of justice.
Realizing the Wagon Lit conductor must have been privy to the plot, Poirot is faced with thirteen persons connected with the case—on is innocent. Poirot concludes that Princess Andrenyi is innocent and her husband took her place. Poirot lists the identities of the passengers and finally comes to Mrs. Hubbard and reveals her true identity. She is Linda Arden, the famous actress and mother of Sonia Armstrong.
Mrs. Hubbard tells Poirot, "I always fancied myself in comedy parts." Mrs. Hubbard explains the entire plot to Poirot. The sentence of death that Cassetti had escaped had to be carried out to prevent him from attacking other children. They were only carrying out his sentence. Mrs. Hubbard asks Poirot if he will just convict her and leave the others out of it.
Poirot asks the Dr. Constantine and M. Bouc what they think. M. Bouc suggests the first solution is put through to the police when the train arrives at the station. The Doctor admits he may have made "one or two fantastic suggestions."
Hercule Poirot, private detective, is the moral authority in Murder on the Orient Express. Because he is a private detective, Poirot is able to work outside the law and decide who is guilty in a case without the state's guidelines. Especially aboard the train, where Poirot is apparently the only figure of justice, his authority is supreme—there is no one to question him. The policemen will meet the train at the conclusion of the journey; until the train arrives in London it is free of established law. A set of morals, different than English or American law, is established clearly by the conclusion of the novel.
By this new set of morals, the Armstrongs are allowed to kill Ratchett and not receive punishment for the crime. Because they had a "jury" of twelve murderers, the crime is permissible. Arbuthnot and Poirot discuss trial by jury; however, the type of jury that Arbuthnot refers to is not the Western sort. When Arbuthnot tells Poirot that he thinks Ratchettt should have been "hanged-or electrocuted" for his crimes, Poirot asks him if he prefers "private vengeance" to law and order. Colonel Arbuthnot tells Poirot that there can't be "blood feuds," but "trial by jury is a sound system." Colonel Arbuthnot draws no connection between established State law and trial by jury, the jury systems is rather an extension of "private vengeance." The "sound system" Arbuthnot talks about is the system he and other friends and family of the Armstrong family used to kill Ratchett—a group of twelve deciding the fate of one man. This "system" is law and order separate from the law and order of the state.
Poirot apparently understands what Arbuthnot means by this statement, he tells Arbuthnot, "I am sure that would be your view." Although Poirot does not know who exactly killed Ratchett at this point, Poirot is sure that whoever murdered Ratchett felt justified in the killing. The clue found in the bedroom specifically associates the Armstrongs with the crime; whoever killed Ratchett was somehow involved with the Armstrong family or sympathetic to their plight. The killer was certainly avenging the death of young Daisy Armstrong. Arbuthnot's statement about the "trial by jury" essentially gives away the motive and justification for the crime and also provides the moral basis for murder.
In the conclusion of the novel, Linda Arden explains the family's justification for murder. She tells Poirot that the family was just carrying out Society's condemnation of Ratchett: "It wasn't only that he was responsible for my daughter's death and her child's and that of the other child who might have been alive and happy now...there had been other children kidnapped before Daisy...there might be others in the future...we were only carrying out the sentence." It is interesting that Linda Arden states that law and society had already sentenced Ratchett, but were unable to carry out that sentence and it was up to the Armstrong family to follow through. Linda Arden and the family by committing the murder, suggest that law is ineffectual. Not only does this undermine state law, but heightens the power of the individual to "right" society and further empowers the private detective as a moral authority-to decide if crime like these are criminal at all. Murder on the Orient Express is a morality novel because it specifically judges and defines the morality of murder.