The St Davids colliery, which had been the original purpose of the first Llanelly Railroad and Dock Company, was no longer the principal location for coal extraction in the area, and the incline operation was inconvenient.
To give good access for colliery traffic, there was a triangular junction there, so that coal traffic from the Amman valley could turn east on to the new line; at the same time express passenger trains (particularly Fishguard boat trains) could avoid Swansea using the line.