The Gorseddau Junction and Portmadoc Railway is a defunct Welshtramway.
The GJ&PR was a 2 ft (610 mm) gauge railway connecting the slate quarries of Cwm Pennant with the wharves at Porthmadog harbour. It was built in 1872, partly as a conversion of the earlier 3 ft (914 mm) gauge Gorseddau Tramway, which in itself had incorporated the even earlier 3 ft (914 mm) gauge Tremadoc Tramway. It opened to mineral and goods traffic in 1875.
The main line followed the route of the original Gorseddau Tramway from Porthmadog through Tremadoc, Penmorfa and Ynys-y-Pandy to Gorseddau quarry. On conversion, the line from Porthmadog to Braich-y-bib, just north of Ynys-y-Pandy, was regauged in 2 ft (610 mm). 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. This branch added an additional 5 miles to the length of the railway, for a total length of 13 miles. At Porthmadog the last few hundred yards of the original Gorseddau route were abandoned and traffic was worked to the wharves over the Croesor Tramway. At around the same time the line from Braich-y-bib to Gorseddau quarry had been abandoned. The Prince of Wales quarry supplied most of the traffic for the railway during its existence.
Unlike its predecessor the GJ&PR had a single steam locomotive, a vertical boilered De Winton named Pert, although it continued to use horses as motive power for most of its existence. The locomotive is believed to have been disused after 1878, and to have been sold in 1896 to Glodd-fa'r-Glai Quarry which was connected to the Nantlle Railway.
By 1887 the railway had largely fallen into disuse as the mines and quarries it served failed, and by 1890 single wagons were being hand propelled to Porthmadog. The land the railway ran on was sold in 1897 by which time it had been dismantled. Between about 1903 and 1907, a short section of Gorseddau trackbed between the Cambrian Railways station in Porthmadog and the junction with the Croesor Tramway was again re-used to connect the Moel y Gest quarry tramway via the Croesor and the Festiniog to the wharves. Although the latter tramway closed in 1907 and was re-laid in 1919 as a standard gauge siding of the Cambrian, this section survived until the early 1950s.