Three similar very short Scottish lines serving tourist areas, but none of them in the public eye.
Choice 1. Ballinluig-Aberfeldy
A nifty route that might have headed off a regeneration need in Feldy nowadays, a picturesque small town that's very much in need of a boost.
2. The Mound-Dornoch
3. Dingwall-Strathpeffer
I've always had a soft spot for the Dornoch branch -- would be splendid if it could have survived in preservation (in its entirety). Rather a long way from any likely sizeable pool of volunteer labour; but to some extent, we're indulging fancies / fantasies here -- not constrained by too much boring reality !
Most of the Lostwithiel-Fowey line remains open for china clay freight traffic. The line between Fowey and St Blazey was closed in October 1968, and the trackbed was converted for use by English China Clays International as a road haul route, and is still (2014) in operation. These routes are therefore not available for conversion to "preserved" leisure lines.
As above -- my OP was meant to be more about playing with what-ifs, than the 100% strictly factual
... Reality demonstrably means that while road clay traffic uses the Fowey -- St. Blazey / Par part of the route, it couldn't be a preserved-rail section. A thought generated here, though -- given flexibility and willingness on the part of all concerned, would it in itself be wildly out of the question, for regular preserved passenger services to operate over a line which was still in use by the relevant national outfit, for commercial freight workings? So far as I know (correction welcomed) this has never been done in Great Britain; but does that mean that it couldn't be done? (Some preserved lines have embarked on initiatives toward carrying freight for potential customers located on their lines, but I gather that little has actually come about, re such ventures.)