Spoilers! En voiture, mes amis, unless you've decided to stay a few nights and see the sights rather than board the Wagon Lits.
The first chapter is set on a different line, the Taurus Express, the stops along which read like a summary of recent extended conflict: Kirkuk, Mosul, Baghdad, Aleppo. Here we first meet Hercule Poirot, the mustachioed and bald Belgian with the egg shaped head, Mary Debenham, a governess headed home to England, and Colonel Arbuthnot, headed home to England from years in India.
Poirot intends to stay in Stamboul, but receives a telegraph recalling him to another case. With more difficulty than anticipated, he gets space in the unexpectedly full first class of the Orient Express. He carefully observes the various passengers in first and second class. He initially shares a room with someone he first saw at the hotel in Stamboul he was briefly at, but when an additional carriage is added to the train, an officer of the rail line switches to the new carriage and Poirot gets his own room.
Of course, his neighbor is murdered in the night and the game is afoot. The novel has been out since 1934 or thereabouts and has been made into movies and short TV series and then riffed and homaged to yards past its death. I think we all know the basic premise of the book: Everyone Did It.
Having gotten the major spoiler out of the way, what's it like reading this thing in 2017? Well, I guess the first and most obvious comment would be how thin the motivating crime feels. The standin for the Lindy baby kidnapping -- a ripped from the headlines plot point if ever there was one -- did not age well. Decades after everyone involved in the Lindbergh case died, we now know a fair number of unsavory things about Charles and his feelings about his son, that make the source case seep through in weird ways to the thinly fictionalized version.
The second, and most offensive aspect of the novel is the relentless ... bigotry? Ethnic stereotyping? Racism? Because the "races" in question are all (western) European, and because we ultimately learn that several people are not the "race" they present as but actually someone else enacting their own stereotype of the "race", it's all more than a little weird and creepy.
Completely by accident, I stumbled across a Wikipedia entry about Graham Greene's _Stamboul Train_, which predates this novel by a couple years, is set on the same train, and shares a variety of attributes with this book, but which honestly sounds a helluva lot more interesting and nuanced -- altho who knows how _that_ would hold up if read now.
I would observe that reading this book in the late 1970s / early 1980s, the world was at least marginally recognizable. I had myself been on multi-day train journeys, albeit always in coach. Borders were still enforced in the areas through which the train passed, and it was still difficult to identify a common second language in which to conduct business with a stranger met on a train. Reading this book in 2017, it is difficult not to feel that this book has receded a great deal further in time. Between WW2, the peak of the Soviet era, and the creation of Europe leading to English being adopted essentially throughout the area as a common second language, it just isn't possible to relate to the world of the people on the train. Which is probably the other half of why this has become an increasingly difficult story to adapt to TV and movies. (That's not stopping anyone -- I think Dr. Who did it a few years back, altho it is worth noting their version involved a Mummy.)
Hercule Poirot is a wildly implausible character in so many ways it's hard to know how to enumerate them. The use of stilted English (word order and other grammatical oddities, not to mention word choice) at least on the surface intended to convey that conversations are occurring in French (and yet still dotted with largely useless interjections in French -- but never German, even tho some convos are also conducted in German) probably did once successfully resonate with people accustomed to talking to people for whom English was a second language and who themselves word-for-word translated expressions from their own language of origin.
Finally, the book is just way too clever for its own good. I'll probably update this after our discussion.
ETA: We had a person in the group who knows Swedish well enough to not believe the Swedish characters version of stilted English at all. Our group settled on the usual Agatha Christie observations: characters not really believable or differentiatable, difficult to feel a sense of place, highly contrived plot, etc. It was a nice discussion, but a little short.