The preservation society had long held ambitions to extend the railway along the former mineral extension from Abergynolwyn to the foot of the Alltwyllt incline, and as early as 1959 work had begun to trace the owners of the land that the extension traversed. Planning began in the mid-1960s, but construction did not start until 1968 when the winding house for the Abergynolwyn village incline was demolished. To bring the line up to passenger standards some of the curves needed to be eased, and this required blasting work, as the line runs on a narrow ledge on the hillside at this point. The extension and new station at Nant Gwernol were opened on 22 May 1976 by Wynford Vaughan Thomas who drove in the ceremonial "golden spike" to complete the extension. Despite this official opening ceremony, minor work still remained to bring the extension to the standard required to run regular passenger trains, but regular train services began on 29 May 1976.