Railways like the Bluebell, GCR and SVR have major population centres on their doorsteps to draw volunteers from.
I appreciate this isn't quite relevant to the thread, but this is a slight exaggeration - the reason the Lewes - East Grinstead line closed in the first place is lack of traffic due to not serving any actual places!
Lewes is, indeed, a town.
Barcombe station is right by the village - obviously no trains run there any more
Newick & Chailey was nowhere near either Newick nor Chailey
Sheffield Park was built to serve the Earl of Sheffield - not a massive population nearby
Horsted Keynes station is a mile from the village
West Hoathly does serve a village - ironically not reopened due to resident concerns!
Kingscote isn't near many houses - I don't know the area brilliantly though.
East Grinstead is a town.
That said, I do take your point, there are plenty of towns near enough the Bluebell e.g. Haywards Heath.