It will be interesting to see if any lines that are forced to close because of COVID 19 and have a mainline connection are looked at by the government for reopening for public use.
I suspect the number would small, and it would take a lot of money to bring them up to standard, but st least the track bed would be clear and the infrustruction will be reasonably intact.
Infrastructure is a major problem with this, I suspect.
Most heritage railways have deliberately re-created the signalling of the past (it's heritage, right!) but the cost of it is one of the reasons that these lines closed in the first place.
Track may be in place but is maintained to 25 mph standard (some better, some not so). Even if the track materials were A1 there would still be a program of realignment and recanting to allow the sort of speeds that a public service requires.
Also provision of suitable rolling stock has to be addressed. Bluebell, Mid Hants, and Swanage are all in 3rd rail areas - where would the DMUs come from? And how would they be serviced? In other areas there are not the spare units available. Indeed, there are shortages at present.
And all this without dealing with the issues of line ownership, heritage operation and manning.
Pat