Justice
One of the most striking twists in Murder on the Orient Express is Hercule Poirot's clear preference that, even after all his work determining who killed Ratchett, Ratchett's killers go unpunished and undiscovered. This is because Poirot clearly feels that, while Ratchett's murder was technically illegal, it was not immoral. Indeed, the 12 passengers who stab Ratchett function as an underground justice system of their own, filling a role that was inadequately performed by the actual jury in Cassetti's trial. While that jury was biased in Cassetti's favor because of his power and money, this makeshift one is biased against him because they have firsthand experience with his destructiveness and evil. Christie therefore implies that all justice systems are influenced to some degree by outside forces or self-interest. As a result, Christie implies, legal routes to justice are not necessarily fairer than extralegal ones—and in this particular case, Poirot determines that extralegal justice is an appropriate route.
Deception
At multiple points throughout this novel, Bouc marvels at other characters' ability and willingness to lie. As it turns out, nearly every character is lying—all of them about their role in Ratchett's killing, and some of them about issues as fundamental as their own names and nationalities. Some characters, like Hardman, make a show of coming clean to Poirot and revealing previously hidden truths. Yet these revelations usually just cloak additional levels of deception, and therefore, even if they're true, constitute lies in their own right through omission or manipulation of the facts. In many cases, the characters in this novel lie by pretending to be shallower, less intelligent, or less complex than they truly are. Thus they may try to embody stereotypes of their age, gender, and nationality, in an attempt to hide the emotionally complex motives that drove them to commit murder.
Empire and Colonialism
This book does not take place in Britain, but it is nonetheless studded with reminders of the power held by that country at the height of its imperial powers. Colonel Arbuthnot, for instance, is returning from service in India, the most prized territory of the British empire. English characters have clearly internalized the language of colonialism in their own worldviews and social interactions. For instance, Arbuthnot calls Debenham a pukka sahib, a term of endearment borrowed from Hindi. Poirot remarks that, by using this phrase, Arbuthnot is emphasizing Debenham's upper-class British roots. Thus, here, imperialist attitudes infuse the way that the British speak and relate to one another. While Christie is not overtly critical of the empire, Poirot at the very least seems skeptical of how British people relate to it.
However, British colonialism is only the most overt and official of the various manifestations of European power visible in this book. The very existence of the Orient Express, easily transporting Westerners around the world, is something of a symbol of this power. One character is a missionary, a role that was essential for European colonizers hoping to exert power in communities the world over. Meanwhile, Mrs. Hubbard is shocked by the foreignness she witnesses in Turkey, as if expecting that country to feel just like the Western ones she's accustomed to. Of course, all these characters are performing roles in order to trick Poirot, meaning that they may to an extent be poking fun at imperialist attitudes rather than embodying them.
American Exceptionalism
European empires aren't the only big, international powers at play here. In fact, the book is set against a background of growing American cultural dominance. It's this cultural dominance that forms the background for the novel's action. When Poirot describes his thought process in solving the mystery of Ratchett's murder, he does so by thinking about where, and in what circumstance, such a diverse and strange group of people might be found together. The only answer he comes up with is "America." Therefore, while various European countries are associated with discrete and unchanging attributes (like British stoicism or Italian extravagance) America is seen as a place where these attributes mix, change, and create entirely new combinations. America is portrayed as a site of great danger—it is, after all, where the Armstrong case took place—as well as one of glamor, excitement, and intrigue.
Loyalty
One of the reasons that readers are unlikely to judge Ratchett's murderers too harshly is that they are primarily motivated, not by mere anger or vengefulness, but by loyalty. The first manifestation of this loyalty is towards the dead. Every character bears some attachment to at least one of the people killed, directly or indirectly, by Ratchett/Cassetti. Their original decision to kill Ratchett comes out of this loyalty. For instance, Mrs. Hubbard/Linda Arden gathers together the group of 12 out of loyalty to her dead daughter and granddaughter. However, as the novel goes on, and characters' goals shift from killing Ratchett to deceiving Poirot, their loyalties also shift (or perhaps simply expand). As Poirot investigates, their primary aim becomes protecting their fellow schemers. Of course, the very ties that cause the passengers to passionately protect one another have their basis in shared loyalty to the dead. In a sense, these passengers have taken their love and loyalty to those who are no longer living and transferred it to people who are still alive.
It's this in-group loyalty that makes the case so difficult to solve. The passengers have taken enormous pains to keep each other safe, presenting such an airtight case that no individual can possibly be blamed: they either all escape unscathed, or they go down as one. Some have even taken additional risks to protect one another: Arbuthnot and Dragomiroff consent to dropping clues that incriminate them specifically, while Count Andrenyi actually stabs Ratchett in his wife's place.
Logic and Intelligence
People in Poirot's orbit, such as Dr. Constantine, often wonder whether he is a genius or a madman. This isn't just because of his eccentric personality. It's also because the way he reaches conclusions can feel bizarre and unexpected. At times, he'll act cavalierly about seemingly decisive clues, refusing to accept what seems like an obvious answer. At other times, he'll declare a seemingly outlandish theory, seeming quite sure that it's accurate. This is partly because so much of his process is deeply internal. Yes, he notices clues in the outside world—and once he's come to a conclusion about those clues, he'll test that conclusion by seeing how suspects react to it. But the in-between step, in which he reaches a conclusion about the evidence he's seen, takes place via quiet thinking sessions. Not only does Poirot not use special equipment or evidence in those moments, but he doesn't use his senses at all—instead, he sits still and closes his eyes. Thus, his greatest asset is simply abstract logic of the kind available to any person, including Christie's readers. As such, readers are on equal footing with Poirot and are able to try and solve the mystery alongside him, though his logical abilities are so prodigious that few will be able to do so.
Importantly, Poirot has great emotional intelligence as well. He's very good at reading people's hidden emotions and parsing their motives. He's also able to contextualize their crimes, understanding not just who killed Ratchett, but also why his killers don't deserve to be punished. Indeed, his emotional intelligence helps supplement his logic rather than interfering with it.
Detective Fiction
Strangely enough, one of the themes of this particular detective story is detective fiction itself. Throughout his investigation, Poirot is aware that his murderers aren't just trying to hide the evidence—they're actually trying to craft a narrative of their own, inspired by pop-culture narratives about crime-solving and murder. In fact, the book's murderers are familiar enough with the conventions of the crime genre (from fingerprinting to bombshell clues) that they can use or subvert these conventions.
Their manipulations are revealed as more wildly meta-textual as the book continues. For instance, it originally appears that a panicked culprit has called out in French at the time of Ratchett's murder, pretending to be Ratchett in order to deceive investigators. However, Poirot believes himself to be a step ahead for a period, because he learns that Ratchett spoke no French. As it turns out, though, the criminals actually wanted Poirot to make this connection all along—and Poirot triumphs only by realizing that the entire French-speaking debacle was an elaborate setup. This means, in essence, that the story consists of endless layers of meta-narrative, and that the person who emerges victorious is the one who can best step see through those layers to assess the situation with logic, ignoring the more tempting cliches of the crime genre.
This complexity makes sense, given that Murder on the Orient Express is part of a series of works featuring Hercule Poirot. Therefore, Agatha Christie's intended audience is most likely familiar with crime fiction. Christie must stay a step ahead of these knowledgeable readers, dropping beloved motifs from the genre without using them in a predictable way.
