The Llanelly & Mynydd Mawr Railway was authorised by an Act of Parliament on 19 July 1875. Its purpose was to resurrect the route of the Carmarthenshire Tramroad and once again export coal from the Great Mountain (Mynydd Mawr) through the port of Llanelli.
Although there was an initial enthusiasm for the scheme the capital of £80,000 could not be obtained but the old tramroad was bought under the powers of the Act. By 1880 nothing else was happening and the Act was about to lapse. However, a revival inspired a new Act which came in to force on 2 June 1880. By this time some work had started on building the line including a road bridge at Sandy to replace an old disused level crossing.
It wasn’t an easy line. It climbed from sea-level to 500 feet along a curving 10-mile route. The first revenue traffic started in June 1881. The growth of traffic was slow and after five years only three collieries were connected to the line, Norton’s, Gilfach and Dynant. However, the line’s contractor, John Waddell, who ran the trains, decided that he would open a new mine. His death in 1888 could have brought an end to that plan and possibly a mortal blow for the L&MMR. Fortunately his son, George, maintained his father’s interest in the area and opened the Great Mountain colliery.
By 1889 the line was carrying 70,000 tons of coal. At the Grouping in 1923, when the GWR took over the branch, coal traffic had increased to 400,000 tons. There were no public passenger services on this line. Coaches were used for workers’ trains.
The line carried coal for 100 years with a “super” pit called Cynheidre being its last and sole customer. There were great plans for Cynheidre in the 1950s and 60s. British Railways (Western Region) laid out capital on track re-alignment and a passing loop. New signal boxes at Magpie Grove and Sandy were built to help handle the increased traffic. Cynheidre proved a difficult investment for the National Coal Board and it finally closed in 1989. The branch closed at the same time. The site at Cynheidre is the home of the preservation group The Llanelli & Mynydd Mawr Company Ltd. The line itself is a cycle and footpath.