Nationalisation of the Railways
One of the major reasons for the nationalisation was WW2. During the war the big 4 (Southern, GWR, LMS amd LNER) were virtually natonalised as part of the war effort.
After the end of the war they were in a terrible state and frankly in no financial state to get themselves back into a profitable state again.
Add to that the Post War Labor Government and you have the perfect storm. Clause 4 was in full swing and the mines, railways, docks etc all fell under the State Ownership Umbrella.
Having grown up when BR was still Steam Trains and only a few Diesels and Dr Beeching had not done his hatchet job (some of it was well overdue but in some areas he went far too far) I can say that the railways now are a far better place than back then.