From Roader's Digest: The SABRE Wiki
Pont Briwet is a bridge near Porthmadog. It cuts the corner when travelling from the A487 Penrhyndeudraeth to the A496 for Harlech, shortening the journey by about eight miles. It was recently upgraded to an S2, concrete bridge. Construction began in March 2013, and the new bridge opened on the 13th of July 2015.
Before the upgrade it was a wooden bridge carrying both the single track of the Cambrian coast railway line and an S1 toll road (suitable for light vehicles only), between Penrhyndeudraeth and Llandecwyn. The access to the bridge is manually controlled by two aspect traffic lights, from the toll booth at the bridge's northern end.
Construction began with the railway element of the new bridge, on the downstream side of the existing bridge, and a temporary road bridge on the upstream side. Then the old bridge was demolished and the S2 road element of the new bridge built in its place. Then the temporary road bridge will be demolished.