Still off-topic here but how possible do you think it would be to also put Kitchener-Stouffville underground into a GO tunnel? Where at Union Station could High Speed Rail be accommodated?
I don't think we would even need to go undeground. If we had a real electric regional rail system, we'd be able to reconfigure the existing tracks (or, at least, the trackspace) to run a far greater volume of trains than we currently do.
Here's a cutaway model from the top deck of Berlin's central station, which handles a far greater volume of passengers and train movements than Union station:
As you can see, there are only 6 tracks and 3 platforms but they are quite a bit different from GO train platforms, and so are the trains that serve them.
Of the 6 tracks, 2 are dedicated for the S-bahn regional rail system. Trains arrive every 5 minutes, like a subway, and the train dwells on the high level platform for about a minute allowing people to get on and get off. Of course, the S-bahn looks more like a subway than a GO train with its single floor and multiple doors per car. The remainder of the tracks are used for intercity trains, including high speed ICE trains. All platforms are high floor., elevated a meter off the tracks like a subway or like in Montreal's Central station.
I imagine in Toronto, with electric regional rail replacing the Georgetown-Stoufville (combined), Lakeshore and Milton corridors, we could have something like this:
2 tracks, 1 centre high level, wide platform for Georgetown-Stoufville, Lakeshore, Milton electric regional trains - these would arrive and depart like subways
2 tracks, 1 centre high level, wide platform for the Airport Express (also electric)
2 tracks, 1 centre high level, wide platform for HSR
2 tracks, 2 side, low level, narrow platforms for the remaining GO trains (Barrie, Richmond Hill) and maybe "long distance GO" (Niagara, KW, Peterborough).
2 tracks, 2 side, low level, narrow platforms for remaining VIA/Amtrak.