The Bloor Viaduct was built in 1913, they had the foresight to built it to accommodate a subway.
"It was designed to facilitate mass transit; its upper deck accommodated trams, while both the Don Valley phase and the Rosedale Valley phase
included a lower deck for rail transport, controversial at the time because of its high additional cost. The bridge's designer, Edmund Burke, was able to have his way, and the lower deck eventually proved to save millions of dollars when the Toronto Transit Commission's Bloor-Danforth subway, opened in 1966, was able use the Don Valley phase with no major structural changes to cross the Don River Valley"
http://en.wikipedia.org/wiki/Prince_Edward_ViaductQueens St station has a station for east west subway that was never built,
People in 50's thought there would be more subways in Toronto, a dream that was never finished