At this time there were also movements to revive the old Carmarthenshire Railway; this had been built as a plateway in 1804, bringing down minerals from Cross Hands to a harbour at Llanelly. The
Llanelly Railway and Dock Company had opened a line from Llanelly to serve pits in the Cwmamman area, also reaching Cross Hands and extending to Llandeilo. In 1861 the obtained an act of Parliament, the
Llanelly Railway (New Lines) Act 1861 (
24 & 25 Vict. c. ccxvii), giving authority to extend from Llandeilo to Carmarthen, and the line opened to goods trains in 1864. Coupled with the Carmarthen and Cardigan Railway, these schemes represented a definite competitive threat to the Kidwelly and Llanelly Canal and Tramroad Company, and it submitted a bill to Parliament to convert the obsolescent canal system to a railway. The bill received the royal assent on 5 July 1865 as the
Kidwelly and Burry Port Railway Act 1865 (
28 & 29 Vict. c. ccxviii); the canal company was to change its name to the
Kidwelly and Burry Port Railway Company (K&BPR). It was to absorb Kymer's canal and build miles of new railways: from Burry Port to a junction with the Mountain Branch of the Llanelly Railway and Dock company at
Llanarthney, and a second main line to
Cross Hands Colliery. The authorised capital for the project was £120,000, of which £72,400 represented the purchase price of the canal system; as out-dated technology threatened by viable competitors this was plainly a huge over-valuation. The new railway system was closely connected with the Burry Port Harbour Company, with which it shared many directors, and the decision was taken to combine the two companies. This was done by a further act, the ''''
(29 & 30 Vict. c. v), of 30 April 1866 vesting the Burry Port Harbour in the and changing the name of the combined undertaking to the Burry Port and Gwendreath Valley Railway''. The harbour company's share and loan capital of £85,000 was added to that of the . The company's parliamentary legal advisors included the misspelling
Gwendreath in the deposited documents and it was under that name that the company was authorised. The company seems not to have noticed the error for two years, but the opportunity to correct the matter was not taken in the '''''' (
31 & 32 Vict. c. i), nor in six later acts. A contractor, Frederick Furness, was appointed on 9 July 1868 and he was prepared to take part of the payment for his services in preference shares. The canal was closed and the line was laid as far as possible on the towpath. Under certain bridges the line was laid at a lower level than the towpath, necessitating short downgrades to pass under; nonetheless the headroom at overbridges was very restricted, leading to problems later. At the locks quite steep gradients were employed to accommodate the change of level. The track was laid with flat bottom rails weighing 50 to 60 lbs per yard spiked directly to the sleepers. It had cost £33,841 to build, excluding the cost of acquisition of the canal. The short Carway branch opened about the end of 1870 and that to the Star Colliery at Trimsaran followed in June 1872. The opening of the main line and these branches encouraged the revitalisation of several pits in the area, by reducing the cost of transport of the mineral to market; however this effect was accompanied by some closures as well. The main line was completed to the foot of the Hirwaen Isaf incline, a mile short of Cwm Mawr in 1870. Mason and Elkington Ltd owned a copper works at Burry Port, and acquired the Pool (or Pwll) colliery in 1865. They started converting an old canal tramroad from Burry Port to Pool into a railway; the work took at least until 1867. The same company converted the old Cwm Capel waggonway to a railway in 1876. It was not directly connected to the , but was accessed through the docks and the line. ==Early operation==