Topic-drifting, and of no help re historical fiction (at least, not of the kind sought by the OP !); but
@krus_aragon , your Great-Western-as-at-1902 map is most interesting, in part in a "snapshot of that time" way, in some things which it includes or omits. The GW's cut-off route to the Severn Tunnel and South Wales -- new construction from Wooton Bassett to the junctions near Patchway -- opened in the following year of 1903, is shown as a thick red dashed line. Likewise, though a thin dashed line, the southern end of the route from Cheltenham towards Honeybourne and Stratford-on-Avon: opened (from memory) 1906. The "Titfield Thunderbolt" line is featured only at its far western end, from junction with the Bristol -- Frome line, to Camerton (opened 1882 and, incidentally, abandoned early -- 1925, I think); the Camerton -- Limpley Stoke part, following the route of the Somerset Coal Canal, did not come into being until 1908 -- 10.
A new and slightly bizarre thing to me, shows up near the bottom of the map, towards its right-hand end. Namely: a line south from Pewsey, terminating at Upavon; and one on apparently the same line-of-route, coming up northward from off-map and terminating at Durrington. I think -- hard to tell for absolutely sure from on-screen map copy -- that these are shown as thin dashed, rather than solid, lines. Googling informed me that this must be to do with a scheme which the GWR had, to add to its system a light-railway branch serving military installations on Salisbury Plain: running north -- south, Salisbury -- Amesbury -- Durrington -- Netheravon -- Upavon -- Pewsey. This was administratively "in the works" as at 1898, but never happened; being forestalled by the opening around that date, of the LSWR's branch to Amesbury and Bulford. For who knows what reason, the map chooses to show two separate bits of this putative line's northern section.
Non-GWR trackage: as at 1902, the Weston, Clevedon & Portishead Light Railway was in operation from Weston-Super-Mare to Clevedon; extension on to Portishead not opened till 1907. Nonetheless, the map does not show even the Weston -- Clevedon portion. And it shows the GW's Wantage Road station; but not the Wantage Tramway, though that opened in 1875. One assumes, I suppose, that the GW regarded these light lines, property of independent companies, as too piffling to merit inclusion on its map !