At 8:45, the Orient Express arrives at Belgrade. Poirot gets out to stretch his legs, but, because of the bitter cold, quickly returns to the train. The conductor informs Poirot his luggage has been moved to compartment number one, M. Bouc's carriage. M. Bouc moved to the Athens coach to allow Poirot a spot in first class. Compartment no. one is directly next to Mr. Ratchett and two doors down from Mrs. Hubbard. While returning to his compartment, Poirot is cornered by Mrs. Hubbard. She tells Poirot that she is "dead scared" of Ratchett and tells Poirot she heard Ratchett trying the communicating door between their apartments the night before. While speaking to Mrs. Hubbard in the corridor, McQueen and Arbuthnot pass by. Poirot hears McQueen tell Arbuthnot to come to his carriage to talk about India. After bidding Mrs. Hubbard goodnight, Poirot returns to his carriage, reads for one or two hours and falls asleep. "Some hours later" Poirot awakens to a loud groan or cry close at hand and the immediate sound of a bell ringing. The train had stopped and Poirot assumes it is at a station. The Wagon Lit conductor knocks on Mr. Ratchett's door. From inside a voice replies, "Ce n'est rien. Je ne suis trompe". Satisfied, the conductor moves on down the hall to another door with a light on. Poirot's watch reads twenty-three minutes to one in the morning.
The train still stopped, Poirot has difficulty sleeping in the curious quiet. As Poirot lies in bed he hears the scuffle of slippers in the hallway. Someone in the car is ringing her conductor's bell over and over. When the conductor finally responds, Poirot hears the voice of Mrs. Hubbard who claims there is a man in her compartment. Poirot then rings his own bell and asks the conductor for some water. The conductor informs Poirot that the train has run into a snow bank and may be stuck for several days. After drinking his water and ready for sleep, Poirot hears a large thud in Ratchett's compartment next door. He looks outside his compartment, but only sees a woman in a scarlet kimono walking down the hallway and the conductor making entries in a book at the end of the hallway. At 9:45 AM, the train still stopped, Poirot finally makes his way to the dining car where all the guests are gathered. The passengers are very anxious and worried about making connections and meeting relatives. After breakfast, M. Bouc calls Poirot to his cabin. M. Bouc tells Poirot that Ratchett has been stabbed to death. The coroner, Dr. Constantine, has determined that the crime occurred between midnight and two in the morning. Suicide is ruled out as Ratchett was stabbed ten–fifteen times. The window in Ratchett's compartment was left open, but M. Bouc think this was done purposefully to make someone think the murderer escaped out the window. If the murderer had escaped out the window he would have left tracks in the snow. The door was locked and chained on the inside. It is apparent that the murderer is still on the train in the Stamboul-Calais coach. M. Bouc asks Poirot to take the case and Poirot accepts.
In Chapters 4 and 5 there is an incredible amount of detail, a flood of important events, conversations and insights that help Poirot unravel the Armstrong murder plot. The reader's attention is peaked throughout Chapter 4 because the reader knows someone is going to be killed, probably Ratchett because he has already been pinpointed as the obvious antagonist, and also because the style and format of the writing. Chapter 4 gives Poirot his first list of clues—the only evidence the reader can trust is through Poirot's perspective.
The characters stage a well-rehearsed show for Poirot—Mrs. Hubbard essentially tells Poirot that Ratchett, later revealed to be Cassetti, is a murderer and McQueen asks Arbuthnot to come to his room to talk about India. This "show" is not revealed until the very end of the book. By detailing these events through Poirot's perspective, the reader assumes they are fact—Mrs. Hubbard went to bed and read and Arbuthnot and McQueen discussed India in McQueen's cabin. The surprise ending is heightened because what the reader assumed was truth is, in fact, not true at all. The all-knowledgeable Hercule Poirot was hoodwinked.
The details that Christie labors over, such as Poirot hearing Ratchett washing his hands in the washbasin sometime in the night, make a second reading of Murder on the Orient Express all the more interesting. When one reads the book after knowing the ending, they can trace and the movements of the murderer. Who ever Poirot heard washing their hands during the night was probably not Ratchett, but the murderer. Details such as these, from the perspective of the person next-door, causes the reader to ask, "who was that person next door and what were they washing?"
The set up, the train caught in a snow bank somewhere between Stamboul and London, is another way Christie intensifies the plot. Not only is there a murder, but also the murderer is probably still on board a train stuck in a snowdrift. The setting neatly frames the investigation around the Stamboul- Calais coach, later confirmed by Hardman's evidence that no one entered or left the coach that evening, and makes the situation much more urgent pressing and dangerous. There is no guarantee that the murderer won't strike another passenger.