Capacity from Alto and GO will be a huge issue for the subway
GO could cause a subway issue for inbound arrivals in AM peak, when GO trains arrive. Not as much for PM peak, as the load would be spread out over more trains.
I don't think Alto would be an issue. I don't think there'll be too many passengers arriving from Montreal before 9 AM. Some from Ottawa.
Say, a 4-car 240-person train. At that time the subway is going about every 2 minutes. How long will it take for all the passengers to disembark? 5 minutes?
How many will head south on the subway (as opposed to uber, taxi, heading north). Let's be generous and guess 50%.
Let's assume that 120 passengers gets spread out over 3 trains. That's an extra 40 people per train - or less than 2 passengers per door. Each Line 1 (TR) train has a peak capacity of 1,100 passengers. So about 4% passenger increase?
Another way of putting that, is that there'd be no impact to TTC if they increased the frequency from 1 train every 2 minutes, to on train ever 1 minute, 59.6 seconds.
It's a rounding error.
GO is much more complicated. Especially as anyone travelling from north of Summerhill to Milton is already going to be going to Union (or at least Yonge-Bloor). So this will both increase and reduce southward travel; probably more increase than decrease. Also, anyone travelling from Summerhil to Kennedy now would head south to Yonge-Bloor and change to Line 2. But here they could now change to GO at Summerhill, and get to Kennedy quicker.
At the same time, would Milton line passengers be changing at Summerhill? Or Dupont? I'd think the latter would be quicker, with it being 8 stops to King on both legs of Line 1. Could also mitigate by having GO stop on the Ontario line, just north of Wynford Drive (for Peterborough traffic).
It would be interesting to see what a transport demand model would show.
Ultimately though, if densification continues, Line 1 is going to need some relief, even if Summerill is a no GO(

). I can see one day that there might be an express subway alongside Line 1 on Yonge (either deep under Yonge, or put bits on Bay, Yonge (between Wellesley and Davisville), Duplex, etc. Stations at Queens Quay, Union, Queen, Dundas, Bloor, Summerhill, Eglinton, Sheppard ...