The success of the Ffestiniog Railway triggered a boom in the construction of narrow-gauge railways, not just in Britain but around the world. In the United Kingdom, the centre of narrow gauge construction was North Wales. The mountains of the north held large quantities of slate and their narrow valleys and steep hillsides meant that the smaller narrow-gauge railways were cost effective. The major slate mining regions at Bethesda, Llanberis, Blaenau Ffestiniog and Corris all developed multiple railways to serve the quarries. Some of these lines, like the Ffestiniog Railway, the Corris Railway and the Talyllyn Railway were common carriers, while others like the Penrhyn Quarry Railway and the Padarn Railway were purely industrial lines.