Within the iron making activity in Merthyr and Dowlais in the early 19th century, Richard Crawshay's Cyfarthfa works was the largest. The Crawshay family home, Cyfarthfa Castle, is now a museum. Until the arrival of the Taff Vale Railway in 1841, the Glamorganshire Canal was their principal means of transport. This was fed by tramroads. The principal relic is the Pontycafnau Bridge, of 1793, built over the river Taff. Built of cast iron, it is the world's first iron bridge and is a scheduled Ancient Monument.

The most famous tramroad in the area is the Merthyr Tramroad which was built from the Penydarren ironworks when the owners fell out with the Crawshays who controlled the Glamorganshire Canal. Its fame is derived from the trials in 1804 of Trevithick's steam engine. Much of the track-bed remains with stone sleeper blocks, and two bridges over the Taff at Quaker's yard survive, Greenfield Bridge and Victoria Bridge, both dating from a rebuild in 1815. At the northern end near the site of the Plymouth Ironworks, the southern portal of a tunnel is preserved.

In 1853 the Vale of Neath Railway arrived in Merthyr from the west and crossed the canal over the impressive Rhydycar bridge by Brunel, consisting of three arches, the centre of 12 m (40 ft.)width and two outside arches intended for the towpath. In 1867 the Brecon & Merthyr Railway arrived from the north making a big curve around the west of the town. It crossed two impressive viaducts, Cefn Coed y Cwmmer, with its 15 arches, 260 m. (256 Yds) long and at a height of 37 m. (122 ft.), and Pontsarn, smaller with seven arches.