Seeing as mines have been mentioned in this thread...
Railtrack to Porthmadoc then Ffestiniog Railway to Tanygrisiau which is almost as close as you can get a car to Cwmorthin Slate Mine and to the Croesor-Rhosydd through trip.
Railtrack to Aberystwyth then Vale of Rheidol Railway to Rheidol Falls Station where some of the Rheidol United Mine levels are literally alongside the railway track.
Railtrack to Porthmadoc then Ffestiniog Railway to Tanygrisiau which is almost as close as you can get a car to Cwmorthin Slate Mine and to the Croesor-Rhosydd through trip.
Railtrack to Aberystwyth then Vale of Rheidol Railway to Rheidol Falls Station where some of the Rheidol United Mine levels are literally alongside the railway track.