How They Actually Did The Murder
Pfeiffer's Arden is the mastermind behind the plan, bringing together the others in a bid for revenge. They know they need somewhere remote and isolated for the murder where they can't be interrupted, and the Orient Express is decided as the ideal staging ground. MacQueen and Masterman pose as Ratchett's underlings, ensuring that he ends up on the train, while the rest position themselves ready to spring. The basic plan is to drug Ratchett (putting the Countess' prescription in his coffee) and then have all twelve stab him once - to both enact justice and keep the true killing blow a mystery. Pretty simple, really.
Things only really get complicated with the entrance of Poirot: when he's put on the train at the last minute right next to Ratchett's cabin (which was originally booked by the no-show Harris), the murderers need to construct an alternate narrative to distract the detective. After a false commotion to distract Poirot and disorient him to the murder specifics, most of this involves simple lies when his interrogates them - they craft a fake series of events that creates the mystery of the short, high-pitched conductor on the train - but they also drop plentiful false clues like the adjusted time on the stopwatch and rogue guardsman button, as well as hiding evidence in his luggage and allowing him to catch a glimpse of Debenham in the red dressing gown. They'd, of course, had to have been secretive even without Hercule next door due to the train's crew, but it was only really his involvement that led to them having to so intricately act out and frame the evening's events.
There are some key aspects of the case from the book that aren't in the film. Poirot is clued in that something is wrong by the smelting pot of backgrounds that make up the train's inhabitants - it's so diverse he rationalizes could only happen in America, alluding to a prior meeting - while the express changing time zones leads to a confusion over when the murder took place. There are other subtle changes (the guardsman is one of the killers originally) but the ethos of the killing is the same. What differs a little is the resolution.
Why Poirot Decides To Let Them Go
Perhaps a bigger shocker than the baker's dozen of killers is that Poirot lets them get away with it; he tells the first, false story of a stowaway to the authorities who buy it because, well, he's Poirot. This feels like a major betrayal of everything he stands for - especially Branagh's fastidious take - but really represents an evolution of his character and greater appreciation of the moral gray of his chosen area of expertise. Yes, the group has killed a man, but Cassetti's acts also left them damaged and emotionally scarred. This isn't a simple act of vigilante justice - an eye for an eye - rather a way for them to perhaps get over the damage he wrought. When Arden tries to commit suicide rather than silence the man who feels compelled to reveal the truth, he realizes the permutability of blame.
Near the start, Poirot says to Ratchett he's of an age where he knows what he likes and what he doesn't; the twist challenges that, showing it's never too late to change and grow. And while at first this seems to irreversibly break the detective's resolve and make him want to step away from his profession - he initially ignores a call for help - after a pause for recollection he's right back in it, ready for a new adventure.
This version of Murder more than any other feels like it wants to be a dissection of Poirot, exploring what makes such a unique individual tick. Doing so with this particular mystery is a very smart entrance to that as it intrinsically undermines his ideals and then sees him come out stronger.
Murder on the Orient Express is a subversion of the murder mystery - a whodunnit where everyone done it - that uses the tropes you expect against you; most of the "evidence" is a distraction by the victims. What Branagh's done is taken that and used it to further explore our detective.