Earlier quoted context omitted.
Tracks are certainly very valuable infrastructure, it's just hard to monetize (and very expensive to build, as you say). So in a way it makes sense for the government to handle it.
The way to monetize is property development and rent around stations. That's how it works in Japan, where private companies sell tickets at a loss and still make profits. In the UK that land got sold off to private equity and the like who are making the real dough while they funnel some of that into donations to the Tory party.
Yes. It's just really hard to do afterwards - having the government expropriate land around the stations and giving it to the railway companies is guaranteed to be wildly expensive and unpopular.
Japan's private passenger railways IIRC make about 50% of their income from ticket sales, the rest comes from leveraging the (very!) high value land around the stations. Many other metropolitan rail systems also get about 50% of the income from tickets, but since the railways don't own the land they have to get the rest through some form of government subsidy.
I think some of the problems with the British privatized rail system is how fragmented it is, with nobody responsible for the whole. And there's lots of government meddling in every interface between all these myriad private companies, providing ample opportunity for corruption and massively misaligned incentives. I suspect it would be better to either nationalize the whole lot under one roof, or then go back to the old pre-nationalization type regional monopolies with a single company owning tracks, trains, stations as well as operating them (a bit like the privatized Japanese railways).