An original aspiration was to reach Fishguard to engender an Irish ferry transit and transatlantic trade, but the latter did not materialise for many years, and never became an important sector of the business.

Over the River Severn at Hock Cliffe between Fretherne and Awre and then follow the coast to Chepstow, Newport, Cardiff, Bridgend, Aberavon, Neath, Swansea, Carmarthen and Fishguard, with a branch line from near Whitland to Pembroke Dock.

The prospectus was published widely as a newspaper advertisement: Surveys have been made of the line of country best adapted for accommodating the extensive traffic of the great mineral basin of South Wales, and the important agricultural interests of that country, as well as the intercourse between the ports of Cork, Waterford, Wicklow, Limerick and Wexford, the inland and manufacturing districts of England and the metropolis.

[1]The line was to pass "within an easy distance of Monmouth" and west of Carmarthen, it was proposed "that the railway shall divide into two branches, one to the naval arsenal of Pembroke and the harbour of Milford Haven Waterway, and the other to Fishguard, whence a communication with the South of Ireland would be effected within six hours.

c. ccxxxix) also authorised a branch from Landore to Swansea, as the original route had not been intended to enter the town centre,[note 1] and another from a point near the later Clarbeston Road station to Haverfordwest.

Agreement was reached with the Great Western Railway that the GWR would lease the SWR on completion of the line to Fishguard, paying 5% on the capital plus half of surplus profits.

[citation needed] However, in 1845 there was a catastrophic failure of the potato crop on which a majority of Irish people depended for a living; this resulted in mass starvation and commercial depression, which deepened the following year.

Fishguard at the time only had significance as a potential ferry port, and the building of the South Wales Railway to that point would obviously be wasted expenditure, so that the directors decided to cut it short.

That they do not openly adopt this wise course, by meeting our advances to that end, is attributable, we believe, partly to the erroneous supposition that the South Wales Company will break down in the attempt to reach Fishguard, and thus be unable to bring the guarantee into operation ...[citation needed] SWR directors who were also GWR directors had had an unhealthy influence on the affairs of the SWR: We recommend the propriety of the retirement of those gentlemen from the Board who represent interests no longer identical with those of the South Wales line proper...[10][citation needed] The first portion of the line was opened on 18 June 1850, from Chepstow to Swansea.

Further negotiation took place and the agreement in March 1851 took the form that the GWR would lease the line for 999 years from the time of completion from Grange Court to Swansea.

[11] In sinking the cylinders to form the piers of the bridge, the workmen had first to pass through 29 feet of blue clay and sand, below which they met with a thin bed of peat containing timber, some solid oak, hazel-nuts, and other similar substances...

The Llanelly Company wished to sell its network to the South Wales Railway, and suggested that the crossing would make its system unworkable: it demanded that the SWR pay compensation equivalent to a purchase.

[note 3] The lease arrangement with the Great Western Railway again became a source of conflict, leading to an arbitration award largely in favour of the GWR.

At Lydney the Severn and Wye Railway, a 3 ft 6in gauge horse-operated line dating from about 1813, crossed the path of the SWR on the level.

This the Taff Vale declined to do, and instead the SWR itself laid narrow gauge track into its own Cardiff station; this was ready in January 1854 and sanctioned for passenger train use in February 1854.

The shortage of capital caused the directors to prioritise the Aberdare route, which opened from Neath on 24 September 1851 for passenger traffic, goods and minerals following in December.

[1] The Neyland terminal was at a remote location, and the nearby settlement named Milford was significant, and local interests promoted a railway as a branch of the SWR, from Johnston.

[16][17] The working arrangement with the Great Western Railway continued to be a source of friction, and at the half yearly meeting of the SWR in February 1860, a letter from the auditors to the directors was read to the meeting: Gentlemen: We think it our duty to call your serious attention to the very unsatisfactory condition of the accounts between the South Wales and Great Western Railway companies.

Your auditors have understood that from time to time attempts have been made to adjust the many items involving large sums in difference between the two companies, but we now conclude they have proved abortive, as… other questions have recently arisen which have so largely added to the disputed amounts as to render any division of profits in our judgment of doubtful prudence.We, therefore, strongly recommend that the agreement between the two companies should be cancelled, and that the South Wales Railway should be worked independently and separately.

Although successful, the line had not fulfilled its potential: the hoped-for heavy mineral traffic from South Wales was in fact largely conveyed by coastal shipping, and the transatlantic passenger business had not materialised at all.

The opening up of deep coal seams in the central part of the South Wales Coalfield from about 1850 had encouraged deep-sea (that is, overseas) export rather than domestic trade.

General commercial social development fed a demand for ordinary passenger and goods traffic, and some coastal towns became important as holiday resorts in the last quarter of the nineteenth century.

The route from London and Bristol to the former South Wales Railway system lay through Gloucester, and the GWR had long suffered from the epithet "the Great Way Round".

The Bristol and South Wales Union Railway provided a rail and ferry connection, but this was not capable of handling bulk minerals, and there was soon a move to cross under the Severn.

[22][23] When the South Wales Railway was constructed, Swansea was placed on a branch line from the through route to Carmarthen; the junction point was at Landore.

[3][27] The combined Great Western Railway brought most of the branch lines under GWR control, and enabled efficiencies of scale to be introduced in time.

The GWR had long been accused of exploiting its near-monopoly of long-distance rail connection, and the grouping enhanced that hostility: In South Wales some of the troubles that were to beset the railways were attributed to the Great Western "takeover".

A major power signalling system was installed at Cardiff, and some rationalisation of the duplicate routes at Briton Ferry and Court Sart, a legacy of the Rhondda and Swansea Bay Railway, were undertaken.

The location was chosen because of the availability of deep water berthing for very large oil tanker ships; a further branch was opened to a new Amoco refinery at Robeston on 20 February 1974.

Finally however rationalisation, chiefly spurred by the introduction of diesel multiple units and the consequent changed pattern of rolling stock servicing, resulted in the closure of the line from Johnston to Neyland on 14 June 1964.