The original line was a 3'6" (nominal) narrow gauge line which ran from quarries in Dyffryn Nantlle to the quayside below Caernarfon Castle, and was opened in 1828. It carried both freight and passengers, and by legend it played a part in the creation of the FR, in that Henry Archer is reputed to have been persuaded to involve himself in building the FR instead of taking a lease on the Nantlle.
Most of the route was converted to standard gauge in the 1860s by the Carnarvonshire Railway which continued the line on south from Penygroes to Afon Wen. The line from Nantlle Station (at Talysarn) to the quarries remained narrow gauge for almost a century, with horse haulage supplanted only towards the very end by an agricultural tractor. One of the last pair of Nantlle horses, Prince and Corwen, attended the FR Centenary of Steam in 1963. The lines were closed in the 1960s.
Parts of the Caernarfon - Dinas section of the WHR are now co used by the WHR and a cycle track operated by Sustrans. There are significant deviations between the 1828 and 1867 alignments, which can be traced in places. Notably, the original bridge over the Afon Gwyrfai at Bontnewydd survives, and the trackbed passes under the Plas Dinas driveway by a short tunnel near Plas Dinas bridge. An intact tunnel can also be seen almost at right angles to the WHR at Coed Helen. It formed part of the Nantlle approach to the town, which crossed the Seiont downstream of the present crossing. The Nantlle's original terminus premises survive within the "Island Site" near Caernarfon Station.