A 9 miles (14 km) route for the line was surveyed, running westwards from tramways linked to the quarries, towards Talysarn and Penygroes.

The chief engineer was the railway pioneer George Stephenson, and construction was overseen by his son Robert, who was assisted by John Gillespie.

[1] The line was run as a toll road with customers charged a fee for their goods to be hauled to their destination.

The railway required significant civil engineering work, including a 400 feet (120 m) long embankment and culvert to the west of Bontnewydd, two tunnels, and a bridge.

The short Coed Helen Tunnel carried the line underneath a road on a gentle gradient, rising from north to south.

Trains typically consisted of four or five wagons, pulled by a single horse travelling at walking pace of around 3 miles per hour (4.8 km/h).

North of Pant, the narrow gauge wagons were unloaded onto the narrow-gauge tracks and hauled by horses along the last 50 chains (1.0 km) to the quayside at Caernarfon Harbour.

[citation needed] In 1870, the LNWR extended the standard gauge line north from Pant to Caernarfon Station where it met a branch built from Bangor in 1852.

In December 1871, the standard gauge tracks were extended onto the quayside, removing the need for transhipment and the isolated Caernarfon section of the Nantlle was abandoned.

Extensive transshipment yards were laid out at 'Nantlle' where slate arriving on the narrow-gauge was loaded into standard-gauge wagons.

[5] The remaining approximately 2 miles (3.2 km) of 3 ft 6 in (1,067 mm) gauge horse-drawn tramway linked Nantlle with the quarries to the west and north.

[7] When the Ffestiniog Railway celebrated its Centenary of Steam on 22 May 1963, a Nantlle horse and handler hauled a demonstration train at Porthmadog.

[12] Some wagons had eyes bolted to the tops of their sides to enable them to be lifted bodily by the Blondins used in some of the quarries.

[13] The wagons were owned by the tramway, rather than the quarries and the many that survived into BR ownership had narrow steel plate bodies, which were mounted between the wheels and bolted to the axles.