From the draft report (linked below):
The Canadian Pacific Railway Bonnybrook Bridge consists of two side-by-side spans which are crossed by four railway tracks. One span is a 1912 pony-truss structure 400-feet long that carries three tracks. The second span is of equal length and dates from the 1960s. The two spans share poured-in–place concrete abutments and piers.
The heritage value of the Canadian Pacific Railway Bonnybrook Bridge derives from its unique status as the only example of a Canadian Pacific Railway (CPR) pony-truss bridge design in Calgary. Truss bridges are made up of straight pieces of wood or steel connected to form triangles, the most structurally stable geometric form. The triangles in the case of a pony-truss bridge are assembled into girders located on either side of the deck. The top cords of the two sides of the bridge are not, however, connected by cross bracing. Pony-truss spans are relatively rare on the entire CPR system. Originally, the bridge consisted of this pony-truss span and was paired with a plate-girder span, built at the same time. These two side-by-side spans shared the same piers and abutments. In the 1960s the plate-girder span was replaced by the current beam bridge.
The Canadian Pacific Railway Bonnybrook Bridge is an important historical component of the CPR mainline in Calgary, and marks the location of a Bow River crossing established during the construction of the CPR in 1883. It represents one of two historical historic crossings of the Bow River in Calgary, along with the CPR bridge in Bowness (1912). The establishment of the CPR mainline - to which this bridge is an integral component - became the basis for the development of Calgary; it is to the mainline that Calgary owes its status as the dominant urban, manufacturing and distribution centre of southern Alberta.
The Canadian Pacific Railway Bonnybrook Bridge holds institutional significance for its historical association to the CPR The CPR dominated the development of Calgary for several decades after the 1883 arrival of the company’s mainline to the community. The company’s later decision to make Calgary a major repair and service facility and branch line terminus encouraged the ongoing development Calgary. The company’s position was unrivaled in Calgary until the arrival of competitors immediately prior to the First World War with the establishment of the Canadian Northern and Grand Trunk Pacific railways. Despite the competition, the CPR remained the most significance railway presence in the city and has been one of the most important sources of economic activity for Calgary.
Link http://calgaryheritage.org/documents/CP ... Bridge.pdf