In August 1911 the Great Western obtained powers to build a new railway of 9¾ miles connecting Gwaun-cae-Gurwen with Felin Fran, where it would make an east-facing junction. The intention was to promote the development of coal mining thereabouts, although this would come with a hefty price tag of around £310,000. The intervention of the First World War meant that the scheme was only partly completed, with a two-mile section opened at the north end in 1922, serving Cwmgorse Colliery, whilst the southernmost three miles was extended to Trebanos Colliery a year later.

The four-mile missing link involved some heavy engineering, including a substantial 100-foot high viaduct, and about a mile of the formation was constructed. At Pontardawe, southbound trains would have encountered a sharp southwesterly curve as they entered a tunnel of 103 yards on a descending gradient of 1:90. This was intended to accommodate a standard gauge single line but work on it was abandoned in the 1920s with only the heading having been driven. It measures approximately 10 feet square.

Although a few small rock falls have occurred over the intervening years, the integrity of the heading is excellent giving that it has been completely unworked for the past 90 years. A brick shed has been built just inside the southern entrance. This appears to date from the 1960/70s but some local reports suggest it was used to store council documents during World War II.

Like the tunnel, the approach cuttings were not excavated to their full operational size and are now choked with vegetation and detritus.