A short section of the original route is in use near Briton Ferry and in the Swansea docks complex.

Although coal and iron had been extracted for some centuries, the first industrialisation of mining in the Afan Valley was in 1811 when Samuel Lettston obtained a lease of land at Cwmavon and in 1819 established a blast furnace there in 1819.

Part of the output was conveyed to Aberavon on the Mynydd Bychan tramroad, a horse-operated wooden waggonway that had been in existence since about 1750.

This was the first trunk railway in South Wales and provided a huge boost to trade there; from that time development of the dock facilities and of industry proceeded rapidly.

The Cardiff system had long suffered from extreme congestion of the railways serving the docks there, and of the berthing accommodation itself, with the result that there were constant complaints from shippers of the delay.

On 12 November 1880, as the Prince of Wales Dock was nearing completion, a public meeting was held in Swansea at which it was proposed to build a railway connecting the upper end of the Rhondda Fawr to Swansea, by making use of the South Wales Mineral Railway between Glyncorrwg and Neath.

A tunnel would be needed at the upper end, and a rope-worked incline on the SWMR would have to be by-passed by means of a deviation.

[7] The bills went to the parliamentary session of 1882 and the R&SBR scheme found preference, but there was stiff opposition from interests in Neath.

They wished to develop their own wharf facilities, and a combination of resentment of the competition from Swansea and discomfort that the swing bridge crossing would impede their own river traffic, resulted in objection to their river being crossed near its mouth,[note 1][8] and the promoters of the R&SBR shortened the scheme to end at Briton Ferry.

c. cci) was passed on 10 August 1882;[9][10] as well as the main line, it authorised branches from Pontrhydyfen to Aberavon and Port Talbot.

This enabled the abandonment of the plans to reach Briton Ferry and to make the tunnel, and the company's capital was reduced accordingly, by the Rhondda and Swansea Bay Railway Act 1886 (49 & 50 Vict.

The first section of line, between Aberavon and Cymmer, was opened for traffic on 2 November 1885,[8] giving improved access to the exceptionally rich coalfields in the Avon Valley.

[13] Land acquisition had been difficult; even now access for the tunnel construction had not been possible, as the Cardiff Times reported: The difficulties in the way of obtaining possession of the land for the construction of the Rhondda tunnel having been to some extent removed, the directors hope they will shortly obtain possession so as to enable them to proceed with the work, which the engineers are confident can now be completed in two years.

In 1890 some short extension lines were authorised by the Rhondda and Swansea Bay Railway Act 1890 (53 & 54 Vict.

c. clxxix) to extend the R&SBR line from Briton Ferry to Swansea, including a swing bridge near Neath, which by now was accepted by the community there.

[12]) In 1929 the government passed the Development (Loan Guarantees and Grants) Act 1929 with the intention of encouraging industrial investment that would reduce the high level of unemployment at the time.

The Great Western Railway took up this scheme, and among other projects, improved the infrastructure in the Swansea and Port Talbot areas.

The main line east from Swansea was exceptionally congested with mineral traffic, impeding the passage of the premium express passenger trains.

[15] Considerable investment was made in the GWR dock facilities at Swansea, especially in modern mechanical handling equipment and adaptation to the changing pattern of mineral exports.

The numerous small-scale marshalling yards at the Burrows (the R&SBR section near Jersey Marine) were modernised, and the residual passenger service over this section was diverted to the GWR (former Swansea and Neath) line running parallel, so that the former R&SBR lines were dedicated to goods and mineral traffic.

The diversion from Riverside freed up the approach of goods traffic at the docks, which had previously conflicted with passenger trains crossing.

[5] The line remained in use from Duffryn Rhondda downwards for mineral traffic, but with the steep decline of colliery work locally, that section closed on 2 November 1964.

[10] The R&SBR Neath branch had been a goods only line since 1935; on 6 September 1965 it closed completely except for a stub serving a private siding (until 30 November 1983).

[5] The R&SBR part of the Bridgend - Cymmer - Treherbert journey was suspended temporarily on 26 February 1968 after distortion was recorded in the lining of Rhondda Tunnel which was closed as a precaution.

The Neath Swing Bridge is on the District Line section; it was fixed about 1982, and is a grade II listed structure.

[16] Note: the line from Dynevor Junction to Swansea Riverside was almost completely subsumed in marshalling sidings in later years.