I volunteered to help lay the WHR track through the Aberglaslyn pass, I can think of worse places to work
Now open to Hafod y Llyn, it is by far britains best narrow gauge line. Number 87 is their newest garratt and I rode on its first fee paying trip through the pass. You really cannot miss this railway. Then when you're done, go to the Festiniog to ride on the double fairlies.
Another I would suggest is the talyllyn. This was the first preserved railway in britain and chuffs its way up the lovely fathew valley. i spent many holidays as a kid on that railway.
I don't really like the narrow gauge railways in england, they are too much like toy trains. Welsh trains are an engineering solution, which to me is much more interesting.