I’m not really sure what the deal here was, but I doubt that the engineers “overlooked” the street car rails. That’s generally the kind of thing one thinks about when building a new bridge. The Wikipedia entry mentions rails for “tram service” but I’m not sure if that’s the same thing.

Cars Use Trackless Bridge

AFTER designing and building of the famous harbor bridge at Sidney, Australia, had been completed engineers realized that they had overlooked the installation of street car rails. As a result one of the city’s important lines was severed and to lay tracks over the completed bridge would have been next to the impossible. An English engineering firm in Liverpool was called on to solve the problem and as a result service has been restored through the use of motorized “trolley trailers.”