Castleton has an excellent bus service, and though not directly on the Hope Valley line, train tickets are accepted on the bus between Castleton and Hope Station (which is!).
It is home to the only Blue John mine in the world as well as four major caves/caverns which tourists can visit. Each has a very different feel, from the natural splendour of Peak Cavern to the disconcerting underground river trip (and well-rehearsed guides' patter) of Speedwell.