The four principal ironworks at Merthyr Tydfil were
Dowlais (built 1759),
Plymouth (built 1763),
Cyfarthfa (built 1765) and
Penydarren (built 1784). Initially the output of these ironworks was carried by packhorse or on carts to Cardiff. In 1794, Cyfarthfa works was linked to Cardiff by the Glamorganshire Canal, the other three ironworks were linked to it by tramways.
Richard Crawshay of Cyfarthfa Ironworks held the controlling interest in the canal company and claimed preferential treatment. The upper heavily locked section of the canal suffered from congestion. A bill for a tramroad from Merthyr to Cardiff was defeated in Parliament in 1799, by opposition from the
Glamorganshire Canal proprietors but the upper section of the proposed line was built anyway, compulsory powers were provided for by provisions in the
Glamorganshire Canal Act 1790 (
30 Geo. 3. c. 82) but were not invoked. The engineer was George Overton and construction began in 1800, under the supervision of Richard Hill of the Plymouth Ironworks. It was completed in 1802. The Dowlais company already had edge rail lines of narrower gauge from their works to near the head of the canal at Merthyr and these were converted to gauge plateways to connect with this new line allowing through running. In 1804, Trevithick's pioneering steam locomotive made a few experimental runs along this line. In 1815, a wooden bridge over the Taff near Quakers Yard collapsed beneath a train carrying iron from Penydarren. The whole train including the horses, the haulier and four other people riding on it fell into the river killing one horse, badly cutting another and injuring two of the people. In 1823, a bill was unsuccessfully promoted to extend the line to Cardiff. It was some of the same promoters who obtained the
Taff Vale Railway Act 1836. Although the Taff Vale Railway opened to Merthyr in 1841 it was not until 1851 that the standard gauge Dowlais Railway was completed allowing through running to its works. Penydarren Ironworks closed in 1859. Plymouth Works did not cease iron production until 1880 but had built a standard gauge line over part of the tramroad in 1871. South of Mount Pleasant the disused tramroad was lifted in about 1890. Only a single photograph of the tramroad has survived, taken in 1862 and showing widening of the Quaker's Yard viaduct. ==Construction==