It's still a very useful line, Graham. My preference is for the Bluebell to reopen the Ardingly line as a third rail preservation line.
Very true [useful line]. Bromley North + Sundridge Park has around a million passenger journeys a year, 20 minute service. But closed on Sunday, and I suspect that still with double track and 2 platforms at Bromley North capable of supporting a 10 minute service. My (incomplete here) thought wondered about the spare capacity and the sharing of resources ... Whitby - Grosmont is very different in character, but an example of an already-running resource share.
For Ardingly, you do have a line previously electrified and an added draw to the Bluebell rather than a fresh setup - and that's a big advantage to "not
another setup" too. As a branch off a preserved line, you don't have the safety issued with dangerous track level electricity at the main operational points for your steam service. You do have extra costs in terms of infrastructure reconstruction and maintenance, and you have drags to get the stock online and off. You can probably use less updated / less reliable / lower top speed stock too.
Two very different approaches ... with other approaches being something that's little more that a train-length test track somewhere, running preserved electric multiple units as carriage stock (already happening in places), or accepting that they'll be static exhibits. Interesting, I would love to take my grandchildren on a 4EPB or a 2HAP, and I suspect there are many, many others with south London roots who might be of similar mind.