Am aware that this thread is essentially about factual, not fictional, railway books; but in the recent absence of much appearing hereon: wonder if I might stray a little, into the "fictional" ballpark -- mods, please "knock on the head" if deemed appropriate.
A "murder / mystery / detective" author with genuine railway links: Freeman Wills Crofts, 1879 -- 1957. This gentleman -- native of Northern Ireland -- trained for the engineering profession, and worked in that capacity for the Midland Railway / LMS Northern Counties Committee rail undertaking in relevant part of Ireland, approx. 1896 -- 1929; after which he converted his hitherto hobby of writing " 'tec" novels, into his main occupation. (There was some discussion of his such works -- including the one which is the subject of this post -- in a thread on this sub-forum, Fictional Railways in Media, which ran for some months in 2022: that thread now closed -- had that not been so, it's thereon that I would have made this post.)
The novel by Crofts, concerned (which I lately happened on, largely by chance): is Inspector French and Sir John Magills's Last Journey (how's that for a clunky title?), published 1930. The meticulous and astute Inspector Joseph French of Scotland Yard -- chief hero of Crofts's numerous "mystery" novels -- gets involved in the investigation of murky doings (non-political -- by inference, action taking place in the very late 1920s) in the general Belfast -- Larne area, involving the death of the mentioned local magnate Sir John Magill. This thread being basically about "railway", not "fiction-fancy", stuff -- will mostly confine self to that element.
The plot centres around complex and "dodgy" happenings in and around the general "narrow seas" area between Great Britain, and the north-eastern corner of Ireland. Poor old French finds himself seemingly endlessly trogging up and down by rail plus brief sea crossings -- usually involving overnight trains (although not a gricer, he seems to enjoy rail travel well enough -- few better alternatives circa 1929, anyway) -- between London and County Antrim. Interesting, though incidental, rail elements pop up now and again. A minor character -- a witness with potentially significant evidence -- shows up: a loco-cleaner in the NCC's employ, who tells of a chance night-time encounter in course of his doing his job at Larne MPD: tells of his having loco no. 50 to clean -- "she's broad gauge, ye understand, an' I was workin' in the broad gauge shed" -- at that time, NCC 5ft. 3in., and 3ft., both fully active at Larne: no explanation of this stuff furnished by the author -- presumably he put it in "just because": one reckons that 95%-plus of the readership, not into railways or their ramifications in Ireland, would just momentarily wonder, "what's this guy drivelling on about? -- doesn't matter, anyway", and move on.
On the "other island" -- copious and tortuous doings on the Portpatrick & Wigtownshire route, largely involving the overnight Euston -- Stranraer sleeper train. In course of their investigations, characters make a couple of journeys on the Stranraer -- Portpatrick branch (closed 1950, if I have things rightly) -- just mentioned in functional passing, its attractions not gone into. In the tale's complex narrative, much travel happens, by all means other than air: sea-borne characters put in at one point, at Campbeltown. No involvement, regrettably, of the Campbeltown & Machrichanish -- active then, though not for much longer.
I -- not a great detective-fiction buff, save for a couple-or-three favoured authors -- found this novel (rail matters aside) readable, well-enough written, but not compelling: Crofts, certainly a prolific author in this genre, seems generally reckoned by those in the "fancy", as "OK, but not one of the great stars" -- his writing style found competent, but rather humdrum. For me, this tale of the unfortunate Sir John and his villainous clandestine doing-to-death (financial gain, the motive) got so wildly complicated, geographically -- "rail, road and sea" -- and otherwise: super-super-ingenious strategy on part of the "baddies", and its laborious unravelling -- that well before book's finish, I was thinking, "oh, please, just make an end, right now -- I don't care any more".
The paperback copy of the book, which I have on loan from local library -- clearly produced in recent years, though actual date doesn't seem to be in evidence -- has on its front and back covers, an "illustration" (drawing, not photo) of a steam-hauled passenger train: whose locomotive has a very much, non-British aspect -- two domes, headlight in front of chimney: vaguely "Austro-Hungarian"-looking, I feel -- totally unlike anything which ever ran in the British Isles. Standard, I suppose -- the publishers think: "for 99% of the punters, 'a steam train is a steam train is a steam train' -- some of the 1% of rail nerds will whinge, no matter what we do: so, to hell with them". I have some sympathy here, with the publishers ...
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), and goes into great detail of his journeys, namely chasing the various Class 50s that were fading into the limelight.