I've seen this place (at least, I think I have...). It's on private land, right next to the owner's house. I spoke to him years ago, when I and a mate were investigating all and any rock outcrops with a view to climbing on them. There is an easily accessible large open entrance, with a pool just inside. As I remember, he has (or had) his own boat, but I wasn't aware that there was a tunnel beyond the pool. If you're down that way, try dropping by and see if he'll accept a small organised party sometime. He seemed pretty friendly, unless you're a squirrel.
Then again, this may not be the place you're discussing. I don't think it's close enough to the tramway to have any locos inside. But there's definitely a cavernous quarry entrance with a pool.
Cheers,
JohnnyM.
Pain is temporary. Glory is forever. And chicks dig scars...