For about 15 years, the town of Blaenavon – north of Pontypool – was served by two branch lines which approached in opposite directions along the Afon valley. The Monmouthshire Railway & Canal Company arrived from the south in 1854 whilst the London & North Western Railway entered from the north 15 years later. The stations were separated by about 250 yards but the L&NWR’s terminus was 100 feet higher up the hillside.

The two companies came together in the late 1870s as they extended their networks to better serve the collieries on the west side of the valley. The MR&C pushed a circuitious route northwards from Pontnewynydd Junction. Extending south from Blaenavon, the L&NWR’s line was connected to the MR&C by north and south-facing chords, the junction for which was on Garndiffaith Viaduct.

Engineered by John Gardner between 1876-77, the viaduct is 185 yards long and curves sharply to the south on a radius of about 14 chains. To accommodate the two-track junction at its western end, the deck’s width almost doubles from 26 feet at span 4 to 51 feet at the abutment. In total, there are nine 50-foot spans, with refuges cantilevered out at both sides above the third and sixth piers.

The structure is mostly constructed of squared rubble with ashlar impost mouldings. The arches however comprises six rings of engineering brick. The piers are slightly battered and feature supports for the arches’ timber centring at springing level.

Opened in 1878, the Great Western Railway introduced passenger services over the line in 1912, making it easier for miners to travel up and down the valley, but these ceased in 1941 due to the Second World War. Regular traffic serving Blaenavon’s Big Pit ceased on 3rd March 1980 and the railway was taken out of use three months later.

Today the viaduct remains in relatively good order, protected by a Grade II listing and forming part of the National Cycle Network Route 46. However its reopening to rail traffic has become a long-term goal of the Pontypool & Blaenavon Railway, a heritage operation, with a view to sharing the structure with foot and cycle users.

(Jaggery’s photo is taken from Geograph and used under this Creative Commons licence.)