The new railway crossing over the River Sado at Alcacer do Sal in Portugal required a 2.7 km long bridge, comprising two approach viaducts and a main bridge. Steel-concrete composite decks were adopted, the approach viaducts having plate girder decks with 45 and 37.5 m spans, and the main bridge three spans of 160 m each with a box girder deck. The main bridge, with a continuous deck over 480 m, consists of three continuous bowstrings with a single plane of hangers on the bridge axis. The main aspects of the bridge concept design are described and the steel design in particular is discussed.