South Wales played an important part in the development of structural cast-iron. Here in Cyfarthfa one such cast-iron structure is significant, not only for its innovative design and that it carries both a tramroad and water troughs, but that it is also the world's oldest cast-iron railway bridge.

The name Pont-y-Cafnau is derived from its purpose rather than location, in Welsh pont means 'bridge' and cafnau 'troughs' hence, 'bridge of troughs'.

Construction was authorised in January 1793 and was almost certainly the work of the engineer of the Cyfarthfa Ironworks, Watkin George (fl. 1790-1811). It spans the river Taff, immediately below the confluence of the Taff Fawr and Taff Fechan, with a 48ft (14.63 metres) square span and a deck width of 8ft (2.43 metres). The water it carried drove the ironworks waterwheels with one trough incorporated into the tramroad deck whilst another, now long gone, was carried above it. The railway element was the 4ft (1.21 metres) gauge tramroad that carried limestone from the Gurnos quarry to the works.

This, the Gurnos tramroad, ran on the top of the decked-over trough, through which ran water taken from the Taff Fechan. It is supported on each side by cast-iron frames with two raking members dovetailing into a king-post at mid-span. An 'A' frame construction is thus formed with a central kingpost the lower end of the kingpost connected to a horizontal member linking the two rakers. The deck trough is supported by three transverse members between the two frames and under the deck trough. Some secondary bracings were later added to the structure and some early tramroad rails, still fastened in shoes cast into the deck itself, can still be seen. Between 1793 and 1796 George constructed the 606ft (184.70 metres) long wooden Gwynne Water aqueduct which crossed the river above Pont-y-Cafnau itself.

Around 1819-20 a view of the aqueduct was painted which shows the aqueduct's trestle construction supported by the central uprights along with uprights at each end of Pont-y-Cafnau. The Gwynne Water turned the great waterwheel 'Aeolus' which produced a cold air blast to the furnaces but this was discontinued and in the early part of the nineteenth century a steam blowing engine was employed instead.

Next to Pont-y-Cafnau is a building with a waterpower link, the reinforced concrete shell of a water turbine house. The final closure of Cyfarthfa ironworks took place in 1921 and in 1928 the works and the water rights were put up for sale. The engineer of the Merthyr Electric Traction & Lighting Company, Alban Bertie Cousins, saw the opportunity to generate a cheap power source for Merthyr's trams (electricity being steam generated from the start of tram operations in 1901).

Over the years the water course system had evolved and the Taff Fechan water now joined, on the ironworks side of the river, a feeder from the Taff Fawr by means of an inverted siphon across the river. In 1929 Cousins cut the siphon sending the Taff Fawr water across the river into the turbine house where it, and the diverted Taff Fechan feeder, would each drive a turbine. Merthyr's electric trams ceased running in 1939 but hydro-electricity continued to be generated at Pont-y-Cafnau until 1953.

Whilst it no longer carries water Pont-y-Cafnau is open as a foot bridge and, along with the turbine house, is now in the ownership of Merthyr Tydfil Borough Council. The bridge is both a Scheduled Ancient Monument and Grade II listed structure.