Ethel Lina White was the Lee Child of her day, or thereabouts, but not the least irony of her career as a detective story writer in the years between the Wars was that the most famous adaptation of her work didn't even use her title, The Wheel Spins, and chucked a few extra characters into the mix because Hitchcock felt his movie needed more fun in it. Two of them, Charters and Caldicott, were such a big hit that they wound up living on into three other movies; these days they'd probably have got their own franchise and gone around thwarting vast plots armed only with well-thumbed Wisdens, but in the 30s and 40s people had more sense, and they were rolled out as comforting comic relief.
The BBC saw all this, and pondered it hard, and duly decided that the world had been waiting almost eighty years for a faithful adaptation of the original book, though they didn't go to the crazy lengths of keeping the original title. They hired the proverbial metric buttload of English character actors and packed them all off an train journey from somewhere in Yugoslavia to Trieste, all so that one of them could vanish, another one could go nuts looking for her, and the rest of them could, for various complicated reasons, flatly refuse to admit that they knew anything about it.
The girl going nuts was a character called Iris Carr, but the actress' name was the infinitely more suitable Tuppence Middleton, exactly the sort of name all high-spirited gels could be relied on to have in the 1930s, or at least in Agatha Christie, which represents the high water mark of my general knowledge about the 1930s English society (I always assume that the social milieu has been meticulously represented in Christie books, on the shaky basis that the plots are so outrageously unrealistic that she could never have gotten away with them unless everything else was fairly grounded). In what was either a brave moment or a spectacularly bad piece of acting, she's thoroughly unlikeable all the way through, which would probably be even more jarring if it weren't for the fact that no-one else is particularly likeable. At the heart of the original books was the notion that everyone on the train is either positively beastly (if Yugoslav) or kind of a selfish dick (if English). So poor old Iris is surrounded by platoons of people with their own various reasons for being unhelpful, and in principle, it takes the whole movie to figure out who's really out to get her and who just doesn't give a toss.
It turns out that watching a bunch of people being dicks to an unlikeable socialite is, on the one hand, an interesting comment on the way people look after themselves without either malice or sympathy for those around them, and on the other hand, kind of hard to get that invested in. There's no-one to like, and we return to the Master, who had the basic wit to make Iris more likeable, her beau less of a drip, and to add some English caricatures who were fun rather than just cold and self-involved. Poor Ethel Lina White is long dead now, God rest her soul, but if she had any lingering doubts about whether Hitchcock did the right thing by tearing up half her novel, the BBC have just shown her what might have happened if he'd done a Coen Brothers on it and shot everything just like it was on the page.