[ben_w]

It's certainly possible this was caused by who owned what; but I'd just add the decline on the graph begins around the UK's pyrrhic victory in WW1 which IMO marked (in tandem with Irish independence) the beginning of the decline of the British Empire; while the rise at the end is roughly congruent with the increasing wealth from exploitation of the North Sea gas deposits and (depending how much you accept the possibility of noise in the data making it hard to tell exactly which year it changed direction) joining the precursor to the EU.

[revelio]

North Sea came on-stream at the start of the 80s. The rise in rail traffic clearly starts around the time of privatization in ~95 and the huge plunge followed by decline starts around the time of nationalization.

Certainly there were other problems: the nationalization was downstream of the socialization of the British economy between the end of ww2 and Thatcher, and as can be seen rail traffic (a general proxy for economic health) is in steady decline from then until it rebounds slightly in the 80s before taking off again once put in (mostly) private hands in the 90s.

The reason the graph seems to run a few years ahead of the changes is that actually privatizing and nationalizing something on the scale of a national railway takes a few years to implement between politicians floating the idea and the final handover of power, but the effect on people's motivations and incentives begins almost immediately.