The Clydach tunnel is near the Gelifelen tunnel, both are in the Clydach gorge area near -ish Abergavenny/Brynmawr the Clydach tunnel is a twin tunnel set up but one tunnel is bricked up, although couriously there is an access passage between both tunnels.
Both tunnels were built around 1862 and were later upgraded to a twin tunnel set up being originally a single line.