Temperature
One of the first things the narrator tells readers is that the weather outside the train is "freezingly cold," and descriptions of the bitter cold arise again and again. Passengers, when interviewed, repeatedly stress that they did not leave the train for long at its various stops because the bitter cold, wind, and snow. Repeated snow imagery emphasizes the snow's blankness and vastness—for instance, "the snow lay in heavy masses," and a passenger recalls "the snow lying very thick." This contrasts with images of intense heat on the train, such as the "hot stuffiness" of the compartments. This contrast demonstrates the enclosed, inescapable atmosphere on the train, building suspense along with a feeling of claustrophobia.
Poirot's appearance
Most of Poirot's fellow passengers don't immediately perceive him as a threat or a formidable presence, simply because he's so unassuming and even a bit silly-looking. In a brief foray into Debenham's point of view, we learn that her first impression of the detective is of "a little man with an enormous mustache...She had never seen anyone quite so heavily muffed...What an egg-shaped head he had." In fact, she imagines him to be "The sort of little man one could never take seriously." But Poirot's unattractive appearance actually serves to emphasize his intelligence, especially since his methods rely so heavily on pure mental ability. Furthermore, his appearance lets him retain the element of surprise. Suspects feel safe enough with him to let their guard down more than they might with a more intimidating person. Finally, the oddness and distinctiveness of Poirot's looks actually contribute to his iconic status. With his comically big mustache and his egg-shaped head, he's memorable and mascot-like from one book or adaptation to the next.
The recovery of Ratchett's letter
Poirot's greatest clue is a half-destroyed letter found among Ratchett's possessions. The letter reveals Ratchett's real identity and points Poirot towards the killer's motive. The narrator describes the process of recovering the letter slowly and dramatically, letting readers know through imagery that they're witnessing a pivotal moment in the novel. First, "the metal began to glow." Then, "suddenly (Dr. Constantine) saw faint indications of letters. Words formed themselves slowly—words of fire." The image of glowing, animated words makes the phrase written on the paper seem almost magical, despite the fact that Poirot is using a scientific process to read them. Moreover, the fiery words seem to reflect the fiery, passionate feelings that motivated Ratchett's killers to act.
The woman in scarlet
When describing the woman in the red kimono, MacQueen, in particular, seems to have a good time adding extra details to his story. In one of the only uses of olfactory imagery in the novel, he notes that her smell was "rather fruity," and that "you’d smell it a hundred yards away." This detail is quite vivid, but seems entirely gratuitous on MacQueen's part. Some passengers seem to regard killing Ratchett as an unpleasant but important obligation, but MacQueen, who has a playful and energetic personality, seems to actually take some enjoyment in the storytelling and artistry that the situation demands.
