Some interesting responses, particularly the Ffestiniog one. The difference with say the Bluebell is that the railway would not be removed, just upgraded and operated by someone else.
At the time, the Ffestiniog was not operating over that part of the route and, as far as I am aware, whilst having an aspiration and a name to go with it, had no concrete plan to be able to do so. Had it been in operation or in progress, the outcome may have been different, especially as it now is operating to the relevant destination, i.e. in exchange for the compulsory purchase, a funding to divert the line would be needed.
In my opinion, a compulsory purchase of all or part of an existing heritage line would be viewed in the same way i.e. an operating one would need to be adequately compensated, one with unfunded ambitions may not.
As I understand, the same will apply to HS2 i.e. compulsory purchase of an existing house has one price, purchasing land which could at some time in the future have planning permission for said house has another.
It could actually lead to some rather interesting future values e.g. the Bluebell accountants could see a possible future CPO for re-integration into the national network as a semi-tangible asset, indeed they may have already. I can't see the same for e.g. the Ffestiniog, nor for a possible dam accross the Ravenglas and Eskdale.