The original WHR ran from Dinas Junction, three miles outside Caernarfon, to Porthmadog. When it opened in 1923 it incorporated the North Wales Narrow Gauge Railways (NWNGR) and the Porthmadog, Beddgelert and South Snowdon Railway (PBSSR).
The PBSSR had taken over an earlier railway, the Croesor Tramway. This line was horse drawn, bringing slate from the Croesor Valley down to Porthmadog harbour. It opened in 1864 and was pretty much unchanged when the WHR took over.
The PBSSR had great ambitions, being part of a power generating company, North Wales Power & Traction. It intended to electrify the NWNGR and Croesor Tramway and to build a new line joining them. By 1906, much of the trackbed was built, but the money ran out, and the line remained untouched until the WHR completed it in 1923.
The original WHR closed in 1937, but when the Ffestiniog came to re-open it sixty years later they extended it north to Caernarfon on the closed standard gauge trackbed of the LNWR. The LNWR line was itself a rebuild of one of the oldest railways in Britain, the 3' 6" gauge Nantlle tramway, which opened in 1828, making it the oldest public railway west of the Pennines.