[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.