Hildegarde Schmidt, the conductors and the doctor are dismissed. M. Bouc and Poirot are left in the dining car to discuss the evidence of the passengers. The only fact Poirot admits is that Ratchett or Cassetti was stabbed twelve times the last evening. Poirot has three theories on the time he was stabbed: 1:15, supported by the evidence of the watch, Mrs. Hubbard and Hildegarde Schmidt and agrees with the doctor's analysis; the crime was committed later and the evidence was faked; the crime was committed earlier and the evidence was also faked. There is no solid evidence the small dark man with a womanish voice, the man described by Hardman as a murder suspect and by Hildegarde Schmidt as a Wagon Lit conductor, actually exists. Poirot thinks he can trust Hardman's account of himself, but brings into question whether his account of what happened the previous evening is true. The small, dark, womanish man theory is confirmed by Hildegarde Schmidt, the Wagon Lit conductor's button found in Mrs. Hubbard's room and the conductor that Arbuthnot and McQueen describe going past McQueen's compartment. M. Bouc tells Poirot the train has been fully searched and no such man exists. Poirot devises two possible answers: the man is hidden or multiple persons committed the murder. As the men leave to search the passenger's luggage for the red kimono and the Wagon Lit jacket, Mrs. Hubbard bursts in, "It's just too horrible. In my sponge-bag. My sponge-bag! A great knife—all over blood!" The woman faints on M. Bouc's shoulder.
M. Bouc escaped from under Mrs. Hubbard and joined Poirot to inspect Mrs. Hubbard's compartment. As the men arrived, all of the passengers had gathered around Mrs. Hubbard's door. Hanging on the handle of the communicating door, between Ratchett and Mrs. Hubbard's compartment, is a large, rubber sponge-bag. Below the bag, on the floor, where Mrs. Hubbard had dropped it, lay a straight- bladed dagger—"a cheap affair, sham Oriental with an embossed hilt and a tapering blade...stained with patches of what looked like rust." The doctor confirms that the blade could have caused any of Ratchett's wounds. Poirot tries the door handle, but it is locked. Poirot stares at the door bolt, about a foot above the handle. Mrs. Hubbard, recovered from her fainting spell, returns to her compartment. Poirot asks her once again about Greta Ohlsson locking the door. Mrs. Hubbard repeats that Greta Ohlsson tried the door and it was locked. Poirot suggests that Ms. Ohlsson did not really lock the door, but just tried the handle. Because it was bolted on the other side, she assumed it was locked. Poirot is confused because the bolt is above the handle, not under the sponge- bag. Mrs. Hubbard is moved into room in the other coach. Her luggage is searched, but nothing suspicious is recovered.
M. Bouc and Poirot search the passenger's luggage. They find a few interesting items: Colonel Arbuthnot has pipe-cleaners like those found in Ratchett's room, a wet label on Countess Andrenyi's suitcase and the famous scarlet kimono found neatly folded on top of Poirot's own suitcase, doubtless put there by the a defiant killer. While inspecting the luggage, Poirot engages in useful conversation with a vengeful Princess who tells Poirot she loved Sonia Armstrong, mother of the kidnapped three-year old, and Mary Debenham who refuses to tell Poirot about a conversation she had with Colonel Arbuthnot on the train to Stamboul. Poirot asks her about her words to Arbuthnot, "Not now. Not now. When it's all over. When it's behind us." Mary tells Poirot that she cannot reveal what she was referring to "being over," but can solemnly swear she had never laid eyes on Ratchett before she boarded the Orient Express and denies knowing Arbuthnot previously.
Christie typically uses a closed circle setting in her novels. The closed circle, meaning there exists a limiting factor that only allows a certain amount of suspects, is not only more convenient for the writer and the detective, but makes the novel more exciting—anyone of the circle may be the murderer. As seen in Murder on The Orient Express, tempers flare, people are anxious and all of the action is heightened because danger exists—a murderer on board. By using a closed circle setting, spatial and human boundaries are defined. The closed circle also works to isolate the crime and make it different and removed from everyday activity. This separation helps the believability of the crime and circumstance because it is isolated, but also helps to further sanitize the crime—it is nothing that happens in everyday society. The setting allows the focus of the crime to be placed only on the crime and detection.
A train stuck in a snow bank is a particularly good "closed circle" setting. As confirmed by Hardman's evidence, the murderer must be on the Staboul-Calais coach. The reader initially thinks the passengers are nervous and fearful because they think there is a murderer on board. Here Christie works against our assumptions: the closed circle setting typically contains one or two murderers, but the Stamboul-Calais coach has 12. The closed circle is meant to limit the suspects, but in this case the entire closed circle, with the exception of Countess Andrenyi, is guilty. Thus, the closed circle setting works in two ways—it makes the ending more surprising because the reader does not expect all of the characters to be involved in a murder and also makes the solving of the crime more manageable. The crime is more manageable because there are a limited number of people to investigate.
The passengers, to conceal their crime, also work against the closed-circle setting. They attempt to fool Poirot into thinking the murderer boarded the train at Vincovci and escaped out Ratchett's window. Hardman tells Poirot Ratchett told him he had an enemy that was small, womanish and dark—unlike any of the passengers aboard the Stamboul-Calais coach. Hildegarde Schmidt tells Poirot she saw a man in a Wagon Lit Conductor's uniform moving quickly down the hallway that matched Hardman's description. The passengers attempt to create an enemy and a person that entered the train, murdered Ratchett and quickly left. The passengers use the idea of a typical murderer in their favor, "typical murderer" in the sense that there are one or two people guilty. The biggest challenge for Poirot and the reader is overcoming this expectation.