The Llancaiach Branch railway line was a mineral branch line in Glamorganshire, South Wales. It was authorised in 1836 as part of the Taff Vale Railway, and its purpose was to connect collieries at Llancaiach and bring their output to Cardiff for onward shipment. It was designed by Isambard Kingdom Brunel and built on the standard gauge. It opened in 1841 from a junction (later known as Stormstown Junction) with the Merthyr line immediately south of Abercynon (then called Navigation House). It was intended to be horse worked, and included a self-acting rope-worked inclined plane near the junction. The collieries were slow to use the line, preferring their customary use of a tramroad and the Glamorganshire Canal, and the value of the line was diminished when the Taff Vale Extension line, an east-west connecting line belonging to the Newport, Abergavenny and Hereford Railway, intersected it and cut off the colliery connections, and the line became dormant.
In 1878 the Taff Vale Railway tried to reinvigorate the line by building a line by-passing the inclined plane, with the intention of connecting with new collieries at the north end, but access over the Taff Vale Extension line was refused.
In 1884 a new connection from Pont Shon Norton, immediately north of Pontypridd, to Albion Colliery on the east side of the River Taff was opened, and in 1900 this line was extended north to join the Llancaiach line. A passenger train service operated through from Pontypridd to Nelson station, just short of the Taff Vale Extension line, but access to that line was still problematic.
The passenger service was popular at first, but it was discontinued in 1932, and as the coal industry declined, the branch was successively cut back, finally closing completely in 1970.