It opened in 1805 between Tredegar and Nine Mile Point, a location west of Risca, from where the Monmouthshire Canal Company operated a tramroad to Newport.

In 1860 the Sirhowy Railway was incorporated to modernise the tramroad; it followed a similar alignment but with several modifications, and opened in 1863, between Tredegar and Nine Mile Point.

The area of the valleys of south-east Wales had long been notable for the rich mineral resources, especially coal and iron ore, and also limestone.

Transport of the manufactured iron, and of the raw materials, was difficult, requiring pack animals to reach Newport for shipping.

At the end of the eighteenth century, determined efforts were made to improve the situation, culminating in the authorisation in 1792 of the Monmouthshire Canal Navigation Company.

[1][2] Many industrialists supported the works as it would considerably enhance their business, and the canal was opened in 1796, by which time many of the connecting tramroads had already been completed.

Samuel Homfray, Richard Fothergill and Matthew Monkhouse were the co-founders of the Tredegar Ironworks, and a lease of 20 March 1800 from the landowner Sir Charles Morgan granted them not only the right to extract coal and iron ore from his land, but to build a tramroad on it "down the Sirhowy Valley to join the Monmouthshire Canal".

[3][4] The engineer Benjamin Outram was called in by the directors of the Monmouthshire Canal to make recommendations for improving the situation in general.

He reported later in 1800, and so far as the canal itself was concerned, he limited his proposals to the provision of additional reservoir capacity, to deal with the water shortage.

His railway would be a plateway, in which the rails are L-shaped plates, the upstand providing the guidance to plain wagon wheels; the gauge was to be 4 ft 2 in (1,270 mm).

The Monmouthshire Canal would build from there to Pillgwenlly, in Newport, except for a one-mile section through the Tredegar Estate which would be built by Sir Charles Morgan: he would retain the right to charge tolls on the mile of route.

About 40,000 l of this sum was expended by the canal company in consequence of building a bridge [the viaduct at Risca] and some very deep and expensive cutting, whilst the Tredegar iron company completed nearly double the distance at the cost of 30,000 l. Sir Charles Morgan expended 4,000 l upon one mile, but he had some deep cutting, and a double road to make.

The whole line of the road for twenty-four miles is an inclined plane, averaging about the eighth of an inch in the yard [a gradient of 1 in 288], or something more, but that part made by the Tredegar iron company is of somewhat greater declivity than the rest.

[8] Nevertheless, it made daily journeys of 28 miles (45 km) hauling loads of 50 and 60 tons, while reducing the cost of horse power by 35%.

[8] The Sirhowy Tramroad was effectively a monopoly, and it was making good profits; for many years it saw little need to adapt itself to changed circumstances.

Nevertherless the neighbouring Monmouthshire Canal Company, on which the Sirhowy line depended to reach Newport, was extending its network.

[12][13] By the end of 1855 the whole of the Monmouthshire system in the Western Valley (with the exception of the Rassa Tramroad) had been transformed into locomotive worked standard gauge railways.

[12][14] The act also authorised a northward extension of the line to a junction at Nantybwch on the planned Merthyr, Tredegar and Abergavenny Railway, which had received parliamentary approval in 1859.

On 10 March 1864, Captain Rich of the Board of Trade issued a report of his inspection of the line preparatory to the introduction of passenger trains; it was not favourable.

I beg to submit that the Sirhowy Railway cannot by reason of the incompleteness of the works be opened for passenger traffic without danger to the public using the same.

[8] Accordingly, passenger services between Sirhowy and Newport Dock Street finally started on 19 June 1865,[15][19] using the Monmouthshire Nine Mile Point branch.

In 1863 the Sirhowy was continuing to make positive responses but the LNWR faction "as being frustrated by an upstart little line which was failing to grasp the economic opportunities falling into its lap".

The GWR promoted a parliamentary bill in the 1875 session for a new line connecting Nine Mile Point and Caerleon, so by-passing Newport, which was heavily congested at the time.

[25] When the LNWR was absorbed into the LMS at grouping a deal was struck with GWR to allow all traffic on the branch to access Newport Docks directly, albeit with a change of motive power at Nine Mile Point.