The company completed construction of two lines, The first, opened in 1877, was approximately 3 miles (4.8 km) long, running south-east from a junction with the London and North Western Railway's Caernarfon to Afon Wen branch at Dinas Junction, to Bryngwyn.

The North Wales Narrow Gauge Railway was intended to run from Caernarvon via Beddgelert to Portmadoc (where it would link with the Festiniog), with a second and very long line from Beddgelert to Bettws-y-Coed and across a long stretch of open country to Corwen, totaling some 60 miles had these lines been built.

The master plan to link almost all the larger places in north-west Wales by narrow-gauge railway included the following proposed lines: An act of Parliament, the North Wales Narrow Gauge Railways (Extensions, &c.) Act 1885 (48 & 49 Vict.

A further part of the scheme was the North Wales Narrow Gauge Railways (General Undertaking) which planned a route from Porthmadog to Beddgelert, where it would meet the Rhyd Ddu-Beddgelert extension.

A short, separate line from Pwllheli (adjacent to the Cambrian Railways' terminus of that period) to Porthdinllaen was also proposed.

The General Undertaking was authorised by the North Wales Narrow Gauge Railways Act 1872 but officially abandoned in 1876.

The section from Porthmadog to Beddgelert was eventually completed by the Welsh Highland Railway as part of its construction, and utilised parts of the Croesor Tramway and the aborted Portmadoc, Beddgelert and South Snowdon Railway (PBSSR) works.

That company later applied for and received a light railway order to build from Beddgelert to Rhyd Ddu and commenced construction on that section around 1906.

At the October 1887 board meeting of the company, a proposal to extend the line from Dinas Junction to Carnarvon was passed unanimously.

In 1900, the railway company was granted permission for an extension of the main line from South Snowdon to Beddgelert.

In 1910, they approached the London and North Western Railway - who owned the Carnarvon to Afon Wen branch - and negotiations proceeded far enough that the acquisition was announced, but it was not completed.

This effort eventually led to the acquisition of the NWNGRs by the Aluminium Corporation and its incorporation into the Welsh Highland Railway in 1922.