Union Station train shed current has 13 tracks with 3 existing tracks. Assuming a disembark time of 90 seconds and train departure time of 70 seconds, 12 trains, one arriving every 10 seconds, over 2 minute period could be serviced or 360 trains an hour. 12-car consists with 136 seats means a peak of 587,520 passengers per hour. However, you need the signals, etc. to get there. Beyond squeezing headways though, you either need to build a grade-seperated corridor for new platforms or look to station elsewhere.
Supplimenting the cost of a GO ticket with city funds seems like a good way to let spending get more out of control. And isn't selling an asset to pay off debt, then using money meant to pay the debt to pay for subways/railroads the same as just selling Toronto Hydro to pay for part of TransitCity+? Why not do something like an IPO rather than selling the wholesale? If it's not supplimenting the cost, it's all political hot air.
The big thing about transit investment at this point is that it's more of "well, we need that too" than "we can do this instead".