SOB-Sitterviadukt, Railway viaduct in Herisau, Switzerland.

The SOB-Sitterviadukt is a single-track railway bridge of the Südostbahn over the Sitter, 365 metres long and 99 metres above the river, making it Switzerland's highest railway bridge. Its 120-metre riveted steel truss with a downward-curved fish-belly shape, still the longest truss span of any Swiss railway bridge, links stone arch viaducts: four 25-metre arches on the St. Gallen side, and on the Herisau side two 25-metre and five 12-metre arches in a 350-metre curve. The whole bridge climbs at 16 per mille.

The Bodensee-Toggenburg-Bahn built the viaduct between 1908 and 1910; the 920-tonne steel truss was assembled by the Bell machine factory and set on four granite blocks of 13.5 tonnes each. The bridge cost 1.55 million francs. Because of embankment slides southwest of the viaduct, two 12-metre arches were added. In 1920–1922 a tie-rod system with a 2240 kN counter-pressure stabilised the two piers, which had tilted up to 270 millimetres toward each other.

The bridge carries the Voralpen-Express and is a class A Swiss cultural property of national significance (KGS no. 17031). Its steel truss, the longest span of any Swiss railway bridge, is a landmark visible from the Kubel bridge and the St. Galler Brückenweg below.

The best view of the viaduct comes from paths along the Sitter River, where you can see the arches and stonework from below. The location is easily reached by public transport, and nearby walking trails offer different angles to appreciate the structure.

The 920-tonne steel truss was erected on a 97.15-metre wooden scaffold tower built by carpenter Richard Coray between summer 1908 and spring 1909, using 1,650 m³ of timber and weighing nearly 1,400 tonnes. A lift inside carried ten to twelve people up 78.5 metres. In November 1909 the tower and bridge withstood hurricane-force autumn storms.