The older line began running trains in 1803, and was a plateway of about 4 ft (1,219 mm) gauge, with horse traction, for the purpose of bringing minerals from the Mynydd Mawr to the sea for onward shipment at Llanelly Docks.

The decline of coal mining in the area seemed to be reversed when the huge new Cynheidre Pit was established in the 1960s, but the railway capacity enhancements to deal with the expected upsurge in traffic proved to be unnecessary.

In the 1760s the area around Llanelly had considerable colliery activity, and there were five short canals from pits to the sea shore; the heavy mineral was brought to shipping for onward transport, at a time when there were no usable roads or railways.

Wharves were an obvious solution, and one of the first was built in 1795 to serve a canal from pits owned by William Roderick, Thomas Bowen and Margaret Griffiths.

In January 1798 he concluded further leases on the property, and one of these authorised him to construct his own tramroad to connect his coal and ironworks with a new shipping place on the mudflats at Llanelly.

[1][page needed] The gauge seems to have been 4 ft (1,219 mm): the engineer's report of September 1801 stated his intention that the line should be built with "rails laid 4 Feet asunder".

However, Price[page needed] records the recollection of an unnamed "old retainer" on the estate: he wrote "The Mynydd Mawr Railway was made 2 in.

The first section of line was opened for traffic from the ironworks at Cwmddyche to the water's edge at Llanelly in May 1803, a distance of one and a half miles.

A considerable embankment was necessary on Mynydd Mawr itself, which delayed opening there until November 1804 and the line was completed to Gorslas (Cross Hands) in 1805.

On 15 August 1806 a special general meeting was held, at which a report was presented outlining unauthorised branches built by Raby, and the fact that his own wagons had used the line without paying the toll.

The financial criticism of Raby gathered pace and in June 1807 he was obliged to sell most of his industrial property to pay debts.

Technical progress in railways had developed edge rails, which were capable of carrying heavier wheel loads, and it is likely that an edge railway had been built alongside the lower part of the Carmarthenshire alignment before 1832, by which time the now-outmoded Carmarthenshire Tramroad had fallen into disuse north of Felin Foel.

In 1833 the K&LCC considered what extensions could be made, and a report recommended extending the canal to Cwmmawr, although that would involve three inclined planes.

[1][page needed] The state of Llanelly Harbour had long given cause for concern, and by the Burry, Loughor and Lliedi Rivers Navigation Act 1843 (6 & 7 Vict.

In 1872 ideas were revived of building a standard gauge line from Llanelly to Mynydd Mawr, giving improved access to the collieries there.

"A resolution to abandon the Llanelly and Mynydd-Mawr Railway scheme, which was come to in October last, was rescinded by a meeting of shareholders on Wednesday, and it was further agreed to apply to Parliament for an extension of the time given for carrying out the undertaking.

A short branch to Llanelly town was authorised in 1881 by a Board of Trade certificate, but the reality of paying for it when the company's financial resources were so stretched, resulted in its abandonment.

On 6 June 1881 two excursion trains were run up the railway; this was before the formal approval of the Board of Trade inspector had been given, and was contrived by the participants, "1,000 souls", paying for refreshments and getting the travelling free of charge.

"[7] The line was considered formally to be opened from 1 January 1883 although it is evident that Waddell had been carrying goods and mineral traffic on his own account for several months prior to that date.

The line conveyed goods and minerals only; there were depots at Felin Foel, Horeb, Cynheidre, Cwm Blawd, Tumble and Cross Hands.

In 1900 the L&MMR considered this possibility, but the Board of Trade, whose sanction for the conversion would be required, stated that the numerous sharp curves and lack of signalling made the scheme unsuitable.

In 1892 and again in 1895 the reclaimed spit of land leading to the light tower was breached by the sea, further reducing the utility of the rail access to the harbour facilities.

The L&MMR stated that the harbour works had involved the extensive use of their railway in the immediate vicinity and that no payment had been received for the activity; when the L&MMR used the new harbour works, and later the nearby Kings Dock, the dues for that usage were retained as a contra charge, and the claimed debt on either side mounted and eventually rose to tens of thousands of pounds the issue only being resolved by negotiation in June 1922.

Moreover, the size of shipping commonly used increased considerably over this period, and the limitations of sea access to Llanelly docks also reduced the activity there.

From 1950 active planning was taking place for a massive expansion of the Cynheidre Colliery, and the considerable extra traffic on the line was to demand improvements in capacity.

These improvements took the form of realignments to eliminate the worst of the sharp curves; provision of a passing loop at Magpie Grove; and installation of signalling.

The event was formally opened by Nia Griffith MP who took the first ride over a short section of the running line to the north.

In 1965 0-6-0 PT 1607 of Llanelli shed 87F was purchased by the National Coal Board and worked at Cynheidre Colliery until 1969 when it was condemned with a cracked frame, and was subsequently scrapped on site.

Continuing northwards the line passed Sandy Junction, where the Kidwelly and Llanelly Canal and Railway joined from the west.

The 1927 working timetable (reproduced in Price[page needed]) shows the workmen's trains calling at Sandy (terminus), Furnace, Felinfoel, Horeb, Cwmblawd, Tumble, S&R Colliery, and Cross Hands.