Eight miles in length, the Carnarvon & Llanberis Railway gained Parliamentary approval on 14th July 1864 and construction got underway two months later. Work was affected by financial difficulties, resulting in a cessation of activity during 1866. The involvement of the London & North Western Railway brought a resumption in December but it was 1st July 1869 before trains first pressed the sleepers, following an inspection the previous day by Colonel Rich from the Board of Trade.

The line was wholly vested in the L&NWR in July 1870. Tourist traffic increased hugely after the Snowdon Mountain Railway opened in 1897 but, as with many rural branches, goods traffic was dwindling by the late 1950s and the last train ran on 7th September 1964. The track was lifted early the following year.

Half-a-mile south-east of Cwm-y-glo Station, the line had to negotiate a rocky spur which extends down to the shore of Llyn Padarn. This involved the boring of a tunnel, 106 yards in length. Generously sized, it is mostly unlined except for a small section at the east end where a road passes over the trackbed. It appears that widening of this roadway necessitated the construction of a skew bridge which has been pragmatically attached to the original tunnel portal, with its much narrower entrance. Beyond it is a short section of lining, comprising masonry sidewalls and a four-brick thick arch, with the void behind filled by rubble.

The remainder of the tunnel has been left just as it was excavated, revealing the layered local geology and many of the holes drilled by miners for their blasting power. Telegraph wire mounting brackets also feature at a high level on the south side.

There is no built portal at the western end. The approach to the tunnel is impressive and can be enjoyed by walkers and cyclists as part of a local footpath along the lakeside.