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.