Turns out it is the former Toronto Power Generating Station, built in 1906 to supply electricity to the City of Toronto, 75 miles away. Designed by architect E J Lennox, clearly no expense was spared in the decoration. It is an unusual example of the Beaux Art style applied to an industrial building but hey, electricity is the future! This was their solution to making a utility building in what was already a major tourist attraction. It couldn't be an eyesore. In 1922 the generating station was acquired by Ontario Hydro, who operated it till 1974.
But what to do with it now? It's all fenced in and boarded up. Not a church, dance halls are no longer in fashion, my vote is to rejuvenate the turbines, modernize the control systems, put it back to work and open it up for tours. It could be a monument to modern renewable energy, much more dependable and predictable than wind.