The main part runs from Swansea to Carmarthen and Whitland, where it becomes three branches to Fishguard, Milford Haven and Pembroke Dock.

This was shown by the fact that Isambard Kingdom Brunel was the engineer, and the line was laid to the 7 ft (2,134 mm) broad gauge.

As a result, instead of completing the line to the proposed port at Fishguard, the Haverfordwest branch was extended to Neyland where the harbour would cost less.

Through a series of inter-company working agreements, this would have had the effect of giving the London & North Western Railway unrestricted access to west Wales.

In doing this the cost of adding a rail to mix the gauge and installing the necessary junctions at Whitland and Carmarthen was £20,000 to be paid to the Great Western within 18 months of the request.

The Great Western maintained a crossing loop at St Clears for the broad gauge and this caused some hindrance to the Pembroke company.

The Rosebush line was closed during World War I because its rails were shipped to the Western Front in 1917 for use by the British Army.

Pembrokeshire escaped lightly from the 1963 Beeching cuts as none of the remaining three branches – to Fishguard, Milford Haven, and Pembroke Dock via Tenby – were proposed for closure.

Additionally, this tight pathing compromised route performance which can amplify delays and hence impact connections into and out of the long-distance Intercity services between Swansea, Cardiff and London.

This is important since there are many interchange passengers from the Pembroke Dock line (which is mainly served by trains terminating at Swansea) for Cardiff and English destinations.

The double tracking work between Cockett and Dyffryn was completed by July 2013 with a revamped Gowerton railway station having the disused platform brought back into use.

Great Western Railway introduced a new timetable in 2023, significantly increasing the service on the West Wales line.

Their change added 65 trains every week on the line,[7] extending services which previously started or terminated at Swansea to call at Llanelli, Pembrey & Burry Port and Carmarthen.

[15] The improvement work has allowed more trains to stop at Gowerton while decreasing overall travel times, and an increase in the frequency of services between Swansea and Llanelli.