You're missing a huge point: The problem for Yonge services southbound during morning peak is that it would not handle a trainload of arriving RER passengers at Summerhill. Union offers many diversions from a single trainload, onto other trains in some cases, GO buses and if onto the subway at Union, there's the choice of *two directions to get midtown!* That's the equivalent of two subway systems, or one with four tracks. That's why rbt and myself were discussing Spadina, even though there are issues with Spadina too, but at least it's an option to 'spread the load'. If Summerhill becomes a terminus for HFR, for instance, or an RER service from the east that isn't 'run-through' please explain to me how Summerhill is going to handle the load.
Steps will have to be taken to ameliorate that, as I stated prior. An obvious one is to divert a good deal of the *crush* factor from the east down the CP connecting spur onto the Don Valley route into Union with a stop at King/Queen, to allow at least some of that *crush* to dissipate along that corridor to their destinations. That will be more direct than going to Union, then up the subway lines....no?
I love Summerhill, and it WAS CP's main station in Toronto until the connecting spur was built to connect to the then new New Union Station. That spur is being refurbished at this very time by Metrolinx for re-use. Why do you think that might be? As it now is, Summerhill will handle light traffic, maybe even medium and wonderfully so, but it can't handle rush-hour crushes....unless some major infrastructure is added to alleviate it....and I don't want to get into that scenario.