Smardale Gill Viaduct curves its way across the narrow valley of Scandale Beck about three miles west of Kirkby Stephen. Constructed of locally quarried limestone the viaduct has 14 arches of 30 feet span, and a total length of 553 feet. It carried the railway 90 feet over the beck below and was built by Mr Wrigg, a contractor from Preston at a cost of £11,298. Mr Wrigg also built the section of railway from Tebay to just east of the viaduct at a cost of £44,216.
Unlike Podgill and Merrygill viaducts, Smardale Gill was built wide enough for double track, but never carried more than a single line of rails.
The deteriorating state of this viaduct in the 1980s, some 20 years after final closure of the railway, was the impetus to the formation of the Northern Viaduct Trust in 1989. Smardale Gill Viaduct was acquired from the British Railways Board and its restoration completed in 1992.
The approaches along the former trackbed to Smardalegill Viaduct are now a wildlife reserve managed by the Cumbria Wildlife Trust