I don't know Toronto well, but it seems there's a good set of expressways that go around the city, so thru roads could be cut to discourage cutting through the city itself. Replace those with walking and cycling paths and people will likely use the mass transit a lot more.
Basically, anywhere the Metro goes should have limited car traffic. That way a subway isn't needed, walk ability is preserved, and noise downtown would be reduced, making for a much more pleasant downtown.
If cities prioritize cars, trains will be more expensive and slow, and walking will be more dangerous. If cities prioritize either trains or pedestrians, cars are inconvenienced, but trains are cheaper and nicer, and walking is safe. Unfortunately, we in NA picked cars...