I've always been intrigued by the Uppingham branch -- grew up not very far from there, but never travelled on it or saw it in action. (Did see its track still down at Seaton, I think shortly before demolition.) Normal passenger service withdrawn 1960, special trains for the school continued for a few more years, I believe. I fantasised about travelling on such a working -- with special permission, or else sneaking on and pretending to be a pupil; never tried turning the fantasy into action.
I'd reckon that most people -- authors or otherwise -- who don't have a railway enthusiast's level of rail-consciousness, tend to be fairly imprecise about such things: one takes it that for Dahl, Euston "felt" about right, even re regular services if that were in fact the case; and he laboured under no compulsion toward total exactitude in the matter.
Going off at a tangent re thread's topic: but, a bit of an embarrassing confession for me, an elderly male -- I have rather a weakness for reading "chick-lit". There's one particular practitioner of same (in the main, her writing quite intelligent and workmanlike, as things go with that genre) whose romance novels -- set in recent decades -- I enjoy; but she does drop the occasional spectacular clanger concerning rail travel. In one novel, she would have the reader believe in its still being possible in the 1980s, to get to Helston by train. That might be reckoned a slight touch of benign "alternative history"; but there's another, recent, work by this author where the heroine and her family live in the depths of Herefordshire, with frequent sojourns in London. Usually, their rail journeys between home and capital are told of as being, properly, to / from Paddington; but in one instance in the book, a journey home from London is referred to as being from King's Cross ! Yet another book by this lady, has the heroine travelling by rail from Sheffield to Oxford; involving a convenient change of trains at, of all places, Gosport (from the context, between Sheffield and London); whence her train takes her to Paddington, from where there's another prompt connection to Oxford. This last instance feels thoroughly surreal on all fronts. I've had brief thoughts of writing to her via her publisher, politely correcting some of her rail-related oddities; but that does feel to me, like missing the point -- she likely enough considers railway enthusiasts to be "sad" people anyway, and such a letter would reinforce that opinion !