at Abersychan, near Pontypool
Garndiffaith Viaduct is a brickwork railway viaduct that formerly carried the Brynmawr and Blaenavon Railway over the valley at Abersychan in South Wales. It was built by the engineer John Gardiner between 1871-4 to extend the London and North Western Railway line that principally carried coal from Brynmawr and Blaenavon to meet the Great Western Railway at Abersychan and Talywain.
In 1912 the lines were opened to passenger services operated by the GWR as well as mineral trains, making it easier for miners and other workers to travel up and down the valley. This service ceased in 1941 due to the exigencies of the Second World War, but the viaduct remained in use until 1980 when the last mineral train left Blaenavon's Big Pit mine. The track was shortly dismantled by British Railways thereafter.
Today the viaduct has survived into preservation and is in relatively good condition, now forming part of the National Cycle Network Route 46. Reopening the viaduct to railway traffic has become one of the long-term ambitions of the Pontypool and Blaenavon Railway as the viaduct is wide enough to accommodate route-sharing.
Sources of Information
-  Wikipedia