Ratchett's Victim Status (Situational Irony)
At the most elemental level, Ratchett is the victim of the crime in this story, while his murderers are perpetrators. Yet, in a twist of situational irony, readers are actually driven to sympathize with Ratchett's killers rather than with their victim. While one's first instinct may be to see a murder victim as helpless and pitiable, Christie reveals that Ratchett was actually a tormentor of helpless Daisy Armstrong. Therefore, even though he is technically a victim, his own crimes are seen as far more important and defining of his character than the revenge exacted on him.
Poirot's Naïveté (Dramatic Irony)
Even though Hercule Poirot is a character in a series of murder mysteries, and even though readers are well aware from the beginning of the book that he'll be solving another (after all, it's right there in the title) Poirot himself is completely unaware throughout the beginning of the book that anything suspicious is happening. He believes that he's an ordinary passenger on an ordinary train trip. Not only do the other passengers know this isn't true, but readers do as well. Since we all know something that our protagonist doesn't, this is a perfect example of dramatic irony. Later, though, Poirot manages to make up for his naïveté by making sense of the crime long before most readers will manage to do so.
Poirot's sarcasm (Verbal and Situational Irony)
When Constantine and Poirot examine the body, and it becomes clear that this case isn't going to be a simple one, Poirot has a brief outburst in which he seems to simultaneously marvel at and deplore its difficulty. He cries out, “The matter begins to clear itself up wonderfully! The murderer was a man of great strength—he was feeble—it was a woman—it was a right-handed person—it was a left-handed person. Ah! c’est rigolo, tout ça!” This is an instance of verbal irony, since he's sarcastically announcing that the case is straightforward, but it also ends up being a display of situational irony. Poirot's sarcastic statement that the murderer had a variety of contradictory qualities actually, shockingly, ends up being true: the murderer was indeed weak, strong, left-handed, right-handed, etc. simply by virtue of being many different people all at once.
Mrs. Hubbard's Persona (Situational Irony)
While many passengers on the train are exaggerating or faking elements of their personality to a degree, none does so more completely than Mrs. Hubbard. She not only invents a whole new name, story, and personality; she invents one that is completely at odds with her reality. While the real Linda Arden is grief-stricken, serious, and selfless—even offering to shoulder the blame for all the passengers—Mrs. Hubbard is flighty, frivolous, and totally self-absorbed. This makes the revelation of her real identity at the novel's end all the more shocking, and also creates an emotional tone that resonates back through the entire book. Suddenly, Mrs. Hubbard's anxiety, not to mention her obsession with her family, looks like a strategy for dealing with and disguising internal pain.
Bouc and Constantine's Agreement (Verbal Irony)
At the end of the novel, Bouc and Constantine, whom Poirot has employed as a kind of jury, both announce that they fully believe in the first of Poirot's theories rather than the second, even though the second theory is clearly the correct one. These announcements constitute an odd form of verbal irony. While the men aren't exactly being sarcastic, they're also not telling the truth. Rather, they're speaking in a kind of code: by saying that they agree with Poirot's first, incorrect theory, they're implying to the passengers that they sympathize with their motives and don't wish for them to be punished. The irony is increased by the sheer unexpectedness of the situation. After putting so much energy into finding the killer, these two figures abdicate the authority they hold and opt not to act on their hard-won knowledge.
