The Viaduct was designed by Edmund Burke, and it was completed in 1918. What is really special about the viaduct was except the level where cars go through, there's another level under the main deck which the trains could go through. According to Ondaatje, there were also water pipes built in so they can be transported across the city. The train deck was initially criticized to be a waste of money, but later, in 60s, it proved a be a huge save of money, as it was in the Bloor-Danforth line as part of the Toronto subway system.
The viaduct, since its completion, became a hotspot for suicides, rated number two after the San Francisco Golden Gate Bridge in North America. This, however, would have nothing to do with the dramatic event of the nun falling off the Bridge during the construction in Ondaatje's fiction.