Very little information about the South Wales Mineral Railway is readily available, the South Wales Mineral Railway was incorporated on 15th August 1853 as a broad (7ft 0½in) gauge line which was leased to and worked by the Glyncorrwg Coal Company until 1870 and then by its successors, the Glyncorrwg Colliery Co Ltd. It was opened in stages between June 1861 and March 1863, and converted to standard gauge in May 1872.
The SWMR was about twelve miles long from Briton Ferry to Glyncorrwg. Between Briton Ferry and Incline Top was an incline of about 1 in 10. The coal wagons are let down by a wire cable controlled by a braked wheel at the top. From Incline Top to Glyncorrwg the line is a single one, about eleven miles in length, with passing places for trains at Ton Mawr and Cymmer, 3½ and 7¾ miles respectively from Incline Top. At Ton Mawr a connection is made with a branch of the Port Talbot Railway, and at this place is a signal box for working the traffic on to and off that railway. The signalman is appointed by the Glyncorrwg Colliery Co, and the Port Talbot Railway pays half his wages. This signal box is in telephonic communication with Incline Top only. At Cymmer there is a small office and a weighbridge with telegraphic communication to both Incline Top and Glyncorrwg. At this place there is a siding connection with the Great Western Railway. There are three trains daily each way, timed to pass at Cymmer unless special instructions are issued by the traffic manager at Briton Ferry or Glyncorrwg for the trains to be worked otherwise. The gradients are not severe for this class of line, the steepest being 1 in 70, and the trains are not as a rule very heavy.
Some remnents of the incline remain, this can clearly be seen on the OS map.