Went here the other weekend, not managed to find out much info on it yet but will see what else I can find and post it up later. The tunnel is about 700 meters long, and curves round, it has been dug out of solid rock and then lined with stone and brick mainly on the roof and some places on the sides, and the rest is just the rock it was cut out from, what I think is red sand stone. Both ends are pretty wet to get in but it’s all mainly dry when you get in. There are small refuges set in the walls all along the tunnel wall. Pictures are not that good as main flash stopped working and did not have time to mess about with it, so I will go back with someone else as taking some more picture will be easier with two people.
East portal looking out.
East portal looking in.
Looking out to east portal.
Refuge.
Inside tunnel.
Refuge.
West portal.
Inside west portal.
East portal looking out.
East portal looking in.
Looking out to east portal.
Refuge.
Inside tunnel.
Refuge.
West portal.
Inside west portal.
. The name of the tunnel is Torpantau, and it was the highest standard guage rail tunnel in the UK. Best of luck with a return trip. 
