Today being the 50th anniversary of Canada's first subway system -- the Yonge line from Eglinton to Union -- today is also a great day to recommend some of the great material available on the Transit Toronto website. This site is run by transit fans like James Bow, not the TTC itself.
The Subway Routes page is particularly interesting today. On that page you will find articles about the original subway length we are celebrating today, as well as each and every successive expansion.
Elsewhere, James Bow has provided his vision for the long term future of the TTC's subway. The map is fun to look at. My own plan would be slightly different -- I think (so long as we're dreaming) we ought to build the Eglinton subway across town (instead of James' Subway 401) but I love his Downtown Relief Line that runs from the airport to Union Station and back up to Don Mills.
Anyway, this last bit is all just for fun. Not much building of any kind is going to take place anytime soon. Finding the money to keep the roof from (literally) caving in is more urgent. But, if any building does take place, you'll be able to read about the plans on Transit Toronto. Plenty of TTC reports are already online.