I was quite prepared to dislike the Llanberis Lake Railway (Rheilffordd Llyn Padarn). I though it was just another narrow-gauge line built on a standard-gauge track bed to catch the tourists.
In fact, the engines all came from the nearby Dinorwic Quarry were they worked the upper levels on an extensive system of narrow-gauge lines, and the track bed (although converted from the original 4ft gauge to 1ft 11½in) was the one originally used to transport the slates from the quarry to the sea at Port Dinorwic.
Given that neither line ever officially carried passengers, and the practical difficulties of trying to operate a tourist line in the upper levels of a quarry, the Llanberis Lake Railway seems a justifiable compromise.
Recently extended along the shore of Llyn Peris to Llanberis itself, the railway can now offer a real, albeit short, journey from the Town to the Museum, in addition to the "there and back" ride, it was when I visited.
Check the Rheilffordd Llyn Padarn website for opening times etc.