 On conversion, the line from Porthmadog to Braich-y-bib, just north of Ynys-y-Pandy, was regauged to .A new extension was added from Braich-y-bib.This led west along the Cwm Pennant before heading north to Cwm Trwsgl where inclines served the Prince of Wales and Dol-ifan-Gethin slate quarries and the Cwm Dywfor copper and lead mine.