I also think the NNR has it about right - a long enough return ride for the average customer, interesting places at each end to look around, good scenery and some decent gradients. Yes it's a bit out on a limb in terms of getting to it, but it's a very popular, year round, holiday area so a fair amount of custom comes from people staying in the area.
At the other end of the spectrum, having been to a few galas there, I think the West Somerset Railway is a bit too long, at 20 miles and around 1 hour 25 minutes to ride the whole length.