The success of the Llandegai Tramway encouraged the owners of the Penrhyn quarry to consider a similar tramway from their slate quarry to Porth Penrhyn.
The plan subsumed the existing tramway into a longer railroad that connected Bethesda to the sea.

Construction started on 2 September 1800, relaying and extending the Llandegai Tramway, with the first slate train traveling on 25 June 1801.
The new railway was also 1 ft 11 ¹⁄₂ in (597 mm) gauge.
The connection to the Llandegai flint mill continued to be used until 1831.
The railroad was operated by horse power along with gravity and three balanced inclines – "Port" (sometimes called "Marchogion") near Llandegai, which was inherited from the Llandegai Tramway, "Dinas" north east of Tregarth and "Cilgeraint" a short distance north of Coed-y-Parc workshops in Bethesda.
The longest extended for 220 yards.
With hindsight this seems hard work, but the overall gain was remarkable, shown not least by the cost of transport falling from 4 shillings per ton to 1 shilling
By the early 1870s the Penrhyn Quarry Railway was no longer able to keep up with the output of the Penrhyn quarry.
By 1874 the decision had been made to replace the railroad with a steam locomotive worked railway without rope-hauled inclines.
Steam locomotives had already been introduced with great success on the quarry's internal railway system and at the nearby Dinorwic Quarry.
The Ffestiniog Railway's example had shown that running a 2 ft gauge line for several miles from quarry to sea was perfectly feasible, and that company's Charles Easton Spooner was retained to survey and map possible routes to the port.
The Penrhyn directors saw every reason not to follow the Padarn Railway's example and build a main line built to a gauge of 4 ft, joining lines in its quarries to lines in its dock both at a much narrower gauge.
All Penrhyn tracks were of the same narrow gauge, so no costly and inefficient internal trans-shipments were needed.
At the end of 1875 the first "mainline" locomotive was delivered to Port Penrhyn and moved to the quarry workshops at Coed y Parc.
The railroad was converted into the new railway in five stages from 1877 to 1878.
The success of the Llandegai Tramway encouraged the owners of the Penrhyn quarry to consider a similar tramway from their slate quarry to Porth Penrhyn. The plan subsumed the existing tramway into a longer railroad that connected Bethesda to the sea. 
Construction started on 2 September 1800, relaying and extending the Llandegai Tramway, with the first slate train traveling on 25 June 1801. The new railway was also 1 ft 11 ¹⁄₂ in (597 mm) gauge. The connection to the Llandegai flint mill continued to be used until 1831. The railroad was operated by horse power along with gravity and three balanced inclines – "Port" (sometimes called "Marchogion") near Llandegai, which was inherited from the Llandegai Tramway, "Dinas" north east of Tregarth and "Cilgeraint" a short distance north of Coed-y-Parc workshops in Bethesda. The longest extended for 220 yards. With hindsight this seems hard work, but the overall gain was remarkable, shown not least by the cost of transport falling from 4 shillings per ton to 1 shilling
By the early 1870s the Penrhyn Quarry Railway was no longer able to keep up with the output of the Penrhyn quarry. By 1874 the decision had been made to replace the railroad with a steam locomotive worked railway without rope-hauled inclines. Steam locomotives had already been introduced with great success on the quarry's internal railway system and at the nearby Dinorwic Quarry. The Ffestiniog Railway's example had shown that running a 2 ft gauge line for several miles from quarry to sea was perfectly feasible, and that company's Charles Easton Spooner was retained to survey and map possible routes to the port. The Penrhyn directors saw every reason not to follow the Padarn Railway's example and build a main line built to a gauge of 4 ft, joining lines in its quarries to lines in its dock both at a much narrower gauge. All Penrhyn tracks were of the same narrow gauge, so no costly and inefficient internal trans-shipments were needed. At the end of 1875 the first "mainline" locomotive was delivered to Port Penrhyn and moved to the quarry workshops at Coed y Parc. The railroad was converted into the new railway in five stages from 1877 to 1878.
Penrhyn Quarry Railway