West portal, 2007. Photo: Ben Salter
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Coordinates: Garnedd Tunnel is approximately half a mile east of Tan y Bwlch station on the Ffestiniog Railway.[route 1] Before 1851, the year it was constructed at a cost of £221 18s 9d, the line veered round the outside of the bluff through which the tunnel was cut. According to Boyd the left-over stone from building Tany-Bwlch Station house in 1896/7 was used to build the West portal. 
During the evening of 21 July 1905 the tunnel's wooden lining was found on fire by one of Hovendon's sons who had to go to Tan y Bwlch to seek assistance. It was midnight by the time help arrived and 4 am before the fire was out. According to the Cambrian News the timber was expected to be replaced with iron sheet. But according to Boyd there was still wooden lining in the tunnel which on the evening of 11 September 1913 caught fire and was burning fiercely as the last down train passed through. Robert Williams fired up a steam locomotive and set out from Boston Lodge with five platelayers. The fire was put out by 1 am. 
The 1836 route is still just visible, being the path used by PW staff to avoid being caught by trains in the tunnel. Though overgrown, it is possible to see the start and end from the current track. Fred Howes says he worked out a plan to relay this outside route with curves aligned so they would allow running at line speed, but there has never been the need to adopt this idea.
The tunnel is approximately 180 feet long. The tunnel is lined only for the first 5 yards from the tunnel mouth though maintenance and enlargement have necessitated a few sections where brick has been laid to strengthen it. There is a geological fault line that runs across the tunnel. The 10 mph speed restriction is to minimise the chance of causing the fault to move.
This tunnel is one of the main restrictions for the loading gauge of the line (the eaves of the carriage roofs). One day a second man decided to go through the tunnel with his hand on the roof of Upnor Castle. He came out the other end with no skin on his knuckles - it is that tight!
A lot of work was done in the tunnel in Spring 1968 before relaying in time for the reopening to Dduallt. The clearance had to be improved to allow the new "Barn" coaches to pass. The floor proved very difficult to dig out and the rock underneath needed to be loosened with blasting. A satisfactory drop of three or four inches in rail level at the critical portals was only achieved with difficulty and then PQR rail was laid and welded. This was the crucial job in allowing reopening to Dduallt on 6th April 1968 to take place.
There is a tale, thought to be apocryphal, that the tunnel was lined with iron plates from U-boat U98 which was broken up in Porthmadog on the foreshore by FR Harbour Station road 6 by a scrap dealer after the First World War. This tale is reported by Boyd.  He maintained that the rails and U-boat sheets were installed in the early 1920s on night shifts so that train services were not interrupted. Fred Howes summarised the evidence in the Heritage Group Journal No. 133 page 32 and says he is sceptical and has seen no sign for it being true. But, he reports that John Dobson recalls walking through the tunnel in 'the early days' and says there were arches at the Porthmadog end with metal plates! Peter Pedr Jarvis in a Facebook post on 3/9/14 says Will Jones told him that the steel arches and plates then holding up the roof in Tunnel Bach were from a U-boat U98. "They rusted away and we had to take them out. Brick now", says Jarvis. For a picture of U98 in Porthmadog see HGJ No. 114 page 39.
Mountaineer approaches Garnedd Tunnel with a train for Blaenau
- Boyd J.I.C. (1975) The Festiniog Railway Vol. 1 page 179.
- Johnson, Peter (2017). Festiniog Railway: The Spooner Era and After 1830 - 1920. Barnsley: Pen & Sword. ISBN 978-1-47382-728-8. OCLC 1003267038. p152
- Boyd J.I.C. (1975) The Festiniog Railway Vol. 1 page 181.
- Fred Howes: The Permanent Way Man, DVD 2013
- "Permanent Way", Ffestiniog Railway Magazine, Issue 041, page(s): 005
- Boyd J.I.C. (1975) The Festiniog Railway Vol 1. page 36.