There has of course been in the thread, plentiful wondering about how much might have been closed how soon, supposing nationalisation had come about in the early 1920s; as ever, speculation about how things might have gone in "different time-lines" is fun, but there's absolutely no way of knowing...
It has for sure, often been observed that relative "backwaters" of Britain's rail system proved invaluable in World War II, for helping to take the overall load of massive wartime freight traffic -- plus the factor of bombing often interrupting things particularly on the main trunk lines. Often cited are the Didcot, Newbury & Southampton; and Midland & South Western Junction; routes, for their very significant role in bringing great amounts of gear manufactured further north, to the south coast around Southampton, for use in the Normandy landings in 1944.
Matthew Engel in his interesting book Eleven Minutes Late -- essentially a "digestible" social history of Britain's railways from inception to the present day, aimed at all-and-sundry rather than at "railway nuts": comments relevantly on this World War II matter, though initially coming at it from a slightly different angle: "...railways which every serious analyst believed should never have been built. And yet the British railway system, constructed with hardly any regard to military considerations, helped defeat Germany whose railways had been planned by generals with precisely that purpose in mind [I feel, rather a sweeping exaggeration and over-generalisation here; but the book, while containing much of interest, has a bit of a tabloid-journalism quality]. Here was the quintessential triumph of British muddling-through...".
