The following information on the bridge is from the "Great Northern Goat's" Website,
In the early 1890's, Jim Hill's Seattle & Montana Railroad crossed through Ballard from Interbay Yard on a large curved wooden trestle located near 14th Ave West and ran along the north shore of Salmon Bay. This first wooden trestle also known as Bridge #4 would eventually have a swing span installed in the middle to accommodate water traffic. 1912 saw the Army Corps of Engineers began preparation for construction of the government locks and the ship canal. This work would necessitate the GN relocating its line over the bay (later the ship canal). Consequently, the GN began construction of the bascule bridge (costing $1,081,836 dollars) over Salmon Bay and the line that now passes through the deep cut under Emerson Street.
Salmon Bay Waterway, Bridge No. 4 was designed by The Strauss Bascule Bridge Co. of Chicago Illinois in 1912. Construction of the structure referred to as a "Strauss Heel-trunnion Bascule Bridge type" on GN blueprints began in 1913 by the Fort Pitt Bridge Works of Pittsburgh, Pennsylvania for the Great Northern Railway. The Bascule designed drawbridge provided a vital link for The Great Northern Railway's Cascade Division, thus connecting Seattle's gateway with Canada to the North and St. Paul to the East. GN records describe the railroad's requirements: "To secure double track on improved alignment, free from grade crossing and to comply with requirements of the U.S. engineers at the crossing of Salmon Bay Waterway, the level of which is to be raised in connection with the Lake Washington Ship Canal."
A 500 ton concrete counterweight is used to lift the 200 foot Bascule span. The counterweight itself was designed with doors located on the south side that open to its interior where heavy rail could be removed during the summer months to compensate for the ties that had dried out from the winter rains. The lift span, the 165 foot truss span to the south, and plate girder approaches on both ends give the bridge a total length of 1,145 feet. At extreme low tide the bridge sits about 50 feet above Salmon Bay's waters. The two towers located on the North and South ends of the superstructure were add ons for electric public utility and signal lines but are no longer used today.