Being laid up at home with mobility issues (hopefully for not too long) , I have ample time for reading so I got out Michael Bonavia's excellent book on the birth of British Railways. An excellent read , concise and thorough read and he was very much in the right place to comment on "high politics" , and commercial reality having served as a senior LNER manager.
Briefly , he makes the point that the political background of the Attlee era made nationalisation possible or desirable - the railways came out of WW2 very tired and in need of a vast renewal catch up etc but by no means financially broken , but would have been better off than they were if they had been commercially paid for actual work done , rather than insulted by no real thanks for their efforts and referred to as a very poor bunch of assets. (not to mention staff depletion , 5 or so years of not much asset renewal and the extreme austerity that was imposed post 1946 say)
The railways could have expanded their road services - both freight and passenger taking advantage of the great post war rise in road traffic by doing much of it in railway owned vehicles (oil rationing permitting) , developed railway / air services with a comprehensive domestic network , and done a great deal to improve freight rail container movements and bigger , faster bulk wagons. (instead the PO wagons commandeered in 1939 were bought off their grateful owners and scrapped........thousands of them), decent freight depots , and plenty of time to study the poorer earning / duplicate and "penetrating" lines and come up with a strategic plan , not the later Beeching plan which he thought was rushed and politically driven as a reaction of the financial and operational problems seen after 1954/5 when the conditions of the railway were exposed.
The playing around with locomotive exchanges . maintenace of steam developed further by the standard classes - often replacing Victorian examples with brand new engines on hopeless lines was the wrong solution (hindsight is a wonderful gift..) , when the real job was developing a proper diesel replacement , prudent electrification and somehow managing resources (like steel for rolling stock and rails) - and employee resources.
Line closures would have been expanded , but probably more focused in the early to mid 1950's with objective , strategic management - no easy task + the later and maybe earlier "Inter City" / block train and domestic Freightliner services would have seen a more robust railway , admittedly in a nation that some tremendous social , economic , industrial changes ........