Tregarth tunnel is located on the former London & North Western Railway (LNWR) Bethesda branch, a 4 mile 26 chain long single track line that linked the village of Bethesda to the Chester and Holyhead main line at Bethesda Junction (52 chains to the east of Bangor). Between Bethesda Junction and Bethesda the line climbed by over 300 feet which meant that there were steep gradients. The Bethesda branch was authorised in 1880. It was engineered by Edward Walter Nealor Wood, C.E. and opened to passenger traffic on 1 July 1884. Goods trains started to run on 1 September 1885.
The tunnel is a single bore and 279 yards in length. In the main it is lined with brick but there are sections that are lined with slate.
The south portal is directly under a road and consists of a skew arch constructed from blue engineering brick. Above the arch there is ledge constructed from sandstone and above it more brick. Immediately to the north of the north portal there is an arched bridge which carried the line over the Afon Ogwen.
The north portal is decorative compared to that at the southern end. It is constructed from stone blocks and includes a dressed stone lintel above the tunnel mouth. Around the arch of the mouth a different colour stone has been used.
An interesting feature of the inside of the tunnel is wooden marker boards that indicate the length. They are numbered 1 to 8 and are placed at intervals of one chain.
The Bethesda branch lost its passenger trains on 3 December 1951 and closed completely on 7 October 1963.
The Tregarth tunnel was found to be in good condition during a Sub Brit site visit on 30 January 2013.
- Les Fifoot
- John Wilson