It is famous as the line, on which Richard Trevithick's experimental locomotive hauled the first train to carry a load (of 10 tons of iron).

Richard Crawshay of Cyfarthfa Ironworks held the controlling interest in the canal company and claimed preferential treatment.

The engineer was George Overton and construction began in 1800, under the supervision of Richard Hill of the Plymouth Ironworks.

[2] In 1815, a wooden bridge over the Taff near Quakers Yard collapsed beneath a train carrying iron from Penydarren.

Plymouth Works did not cease iron production until 1880 but had built a standard gauge line over part of the tramroad in 1871.

There were frequent passing loops, at Plymouth Ironworks the line ran through a tunnel only 8 ft high beneath the charging area of the blast furnaces.

[1] Near Quakers Yard two timber bridges carried the line over the River Taff where it runs through a gorge in a large loop.

In 1829, Stephenson supplied a six-wheeled locomotive with inclined cylinders mounted at the rear for use on the narrower gauge internal lines at Penydarren, it cost £375.

In 1832 it was returned to Stephensons for conversion to a four-wheeled locomotive for use on the Merthyr Tramroad and at the same time the single flue was replaced by 82 copper fire tubes.

In 1832 the Neath Abbey Ironworks supplied a six-wheeled rack and adhesion locomotive weighing 8 tons named "Perseverance" with inclined cylinders and twin chimneys (allowing them to be lowered alongside the boiler to pass through the tunnel at Plymouth).

Another somewhat similar locomotive (but without a geared rack drive) named "Mountaineer" was built in 1833 by the Neath Abbey Co. for the Dowlais Company.