The Great Western Railway took the decision to extend the line, starting from a junction with the Caerau line at Nantyffyllon, and driving a tunnel through the mountain to Cymmer, making a junction there with the South Wales Mineral Railway. The necessary authorising Act was obtained on 1 July 1873, to build from Nantyfyllon to Abergwynfi. The construction involved driving a tunnel in length, and making a large single span bridge crossing the River Afan at Cymmer. The tunnel cost £47,422. The line opened on 1 July 1878 for mineral traffic; passenger services started on 16 July 1880.