Esoteric Happy Ending: Poirot tells the passengers he knows that they all had a hand in killing Casetti and how they did it, but ultimately goes along with Mr Bouc and Dr Constatine's agreement that the first theory - a stranger having jumped onto the train, killed Casetti and left - is what they will give to the police. The entire Armstrong household has spent a long time figuring out this act of revenge and comes off scott-free for killing a man; even if said man deserves it. The novel depicts this as a pretty happy and nonchalant ending. Further adaptations actually went into the ethical question of whether this is or isn't a good idea, as depicted below.
Love It or Hate It: The 2010 televison adaptation with David Suchet falls under this, having a Darker and Edgier tone dealing with Poirot's moral and religious struggles and paying less attention than either the novel or other versions of the story to The Reveal, mostly on the grounds of It Was His Sled. In this version, Poirot is horrified when he finally deduces what actually happened, and the killers openly contemplate murdering him (and the train manager) too to cover up their crime when he announces it, since they could conceal it easily as they are all stuck on a train in the middle of nowhere in the middle of a snowstorm, and both they and Poirot know it. Both are issues that the original novel skips over but probably crossed the minds of most readers when they actually read it, so whether this version gains or loses points for bringing them up is up to the viewer, though neither the identity nor the outcome are substantially changed (the latter is left slightly more ambiguous, but it still seems that Poirot makes the same decision as in the novel, just with much more reluctance). Like most episodes of the series, some changes are made to certain characters and one is omitted.
Possibly also an example of Getting Crap Past the Radar, Greta Ohlsson testifies to accidentally walking into the deceased's compartment in her nightgown and robe, receiving a comment of "You're too old" in response. Now remember why he was killed.
It Was His Sled: Everybody knows what happened, even without reading the book.
Strawman Has a Point: On both sides. Poirot is correct in that people should always try to choose law and order over vigilantism and anarchy. However, the passengers retort that they already relied on lawful means to punish Cassetti, which only resulted in failure and further grief for them — and the 2010 version Retcons him into a high-ranking Mafia member, arguably giving them a "public service" advantage as well.
Took a Level in Jerkass: Poirot in the 2010 adaptation. He almost seems more outraged by Cassetti's murder than that of Daisy's.
Values Dissonance: A mild example, in Poirot's referring in his notes to one of the passengers as an "American subject". Americans find it politically objectionable to be called subjects (to whom?) rather than citizens.
Invoked in the television adaptation when Poirot and a couple of the passengers witness a stoning of an adulteress in Turkey and Poirot excuses it with this trope; it comes back to haunt him later when the girl who saw it and angrily disagreed with him asks him how he could stand back and let that happen but (in this version) is revolted by the vigilante justice of the murderers, despite the Asshole Victim being far more deserving of his fate than the adulteress was. He fails to give himself a satisfactory answer.
The original book had Poirot note, when he was contemplating all of the passengers in the coach and their numerous nationalities, that "only in America" could such a gathering occur. This is used to help him deduce they were all part of the Armstrong household or had connections to the family, and start "casting" them in roles...but not only are some of those role-castings themselves based on outdated views of the nationalities in question, but it becomes rather Harsher in Hindsight considering a) the melting pot view of America has become rather dismissed and fallen out of favor in the years since then and b) thanks to views on immigration changing, it's far less likely such nationalities, or a household containing them, would be as accepted or celebrated today. Even in the time the novel was written, Americans were not nearly as accepting as Christie depicts them (issues with immigrants being a Cyclical Trope, things weren't as bad as in the 1800s, but the stigma was still there), making this another aspect of her Eagleland views. For that matter, many European nations are becoming heterogenous enough that such a mix wouldn't be any less likely there than in the U.S.