 The ironstone mine was not successful, so the tramway was extended to serve a nearby slate quarry, which was owned by the "Bangor & Portmadoc Slate & Slab Co. Ltd."

In 1856, the Bangor & Portmadoc Slate & Slab company requested tenders to extend the line to the Gorseddau slate quarry (known at the time by its anglicised name, Gorsedda), at Glan Bwll.The engineer was James Brunlees, who was based in Manchester.The extended railway, completed in 1857, was known as the Gorsedda Tramway; the correct Welsh form is Gorseddau.