The viaduct is the third largest in Wales and is now a Grade II listed building.
It was designed by Alexander Sutherland in conjunction with Henry Conybeare and partly built by Thomas Savin and John Ward. In early 1866, the project faced disaster when Savin and Ward suffered serious financial and legal difficulties. It was eventually completed with the assistance of Alexander Sutherland. He produced an alternative route into Merthyr, curving the viaduct, so that the railway line avoided property owned by ironmaster Robert Thompson Crawshay. It cost £25,000 to build (equivalent to £2.1 million in 2016).
It consists of 15 arches, each one 39 feet 6 inches wide, and is 770 ft. long with a maximum height of 115 ft. It was planned to be constructed entirely of limestone like the nearby Pontsarn Viaduct but a trade union strike by stonemasons in February 1866 caused the company to buy 800,000 bricks and use bricklayers to complete the 15 arches. It was completed on 29 October 1866, three years after the main line which linked it with Brecon. The last trains travelled over the viaduct in the mid 1960s and it subsequently fell into disrepair. It was refurbished by Merthyr Tydfil County Borough Council with assistance from a grant from the National Lottery. It has now become part of the Taff Trail, route 8 of the National Cycleway.
