I agree privatisation did indeed cause a massive revamp of train usage, and I feel like the nostalgia is sometimes rose-tinted and misplaced because 80s BR by all accounts was pretty bad even in the context of the UK in the 80s.

On the other hand, from about the 90s onwards there was a huge increase in London employment relative to the rest of the country, and also a huge (5x or more over that time) house price increase in London. This pushed a lot of people into the commuter belt. and As you basically cannot drive into London from there, those miles can only be provided by rail.

It would be interesting to know how much of the effect is in the South east, and how much is showing labour migration into a rail-centric region after industrial sectors outside the South-east were dynamited in the 80s.

And privatisation does usually have a cycle: influx of cheap capital in a boom phase, followed by a slow ratching squeeze as you struggle to exceed that great initial growth every year thereafter.