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Rhiw Plas bridge is a concrete bridge, constructed on the site of an earlier bridge, to carry the main Porthmadog to Minffordd road over the railway.[route 1] The former bridge, which was narrow and had sharp turns, was demolished in the late 1950s, the surplus stone and materials being carried up to Tan y Bwlch by rail.
Pending the completed construction of the current bridge, there was a temporary level crossing on the site.
The present bridge was constructed in 1959/60 to ease the awkward double bend in the road, and to increase the capacity of the bridge for road traffic. It opened on 1st June 1960. It had been anticipated that some materials for the construction of the nuclear power station at Trawsfynydd would be brought, by sea, to Portmadoc for onward transfer by road but, in the event, this hardly ever happened.
The rebuilding of the bridge also gave greater clearance, and allowed the building of the larger Centenary stock (Barns). (This location and the Old Moelwyn Tunnel being the defining points for the FR loading gauge in the days of the original company)
Although classed as a bridge, its great length, caused by the skew of the road passing overhead, means that it is normal practice to switch the passenger lights on when running through. On this basis alone, some passengers may believe they have travelled through a tunnel.