I think you're coming at it from the wrong angle. Why should the public sector run any services, unless there's a public interest in doing so? If there is a public interest, like the police or army for example, then sure. But if there is not, then why should the state run it? Why would you care?
Selling off state-owned businesses has raised £100bn+ for the exchequer, which in turn has meant every tax payer in the country is paying less tax than they otherwise would have to do, given our current public spending commitments. So it is intrinsically a good idea, with the caveats of public interest.
And FWIW, I remember what the trains were like in state ownership, and it was horrendous previously. The current situation is far from ideal since costs have been allowed to spiral, but returning it to the dogshit it was before, is no solution.