I will assume the broader definition of "transit" to include buses. Rail GO to places like Haldimand and Brant counties seems to tread on VIA territory and I'm not convinced there is much regular commuter traffic to the GTA from theses areas (to Hamilton, yes). I can see Dufferin and Wellington counties as Guelph and Orangeville are at their extreme south ends. Orillia doesn't have rail. Does this foretell of a GO train to Peterborough?
Perhaps GO and VIA should get together and figure out who is servicing what area.