Sorry but no you just don't need to have another line under Yonge. Toronto has such a barebones subway system that building new lines in other areas is basically always the best option. You don't determine these things just by walking along the street and admiring how much is getting built. You do it by analyzing travel patterns, which is just what the DRL background studies did.
Building a new subway under an existing subway would be a colossal expense, probably more expressive and definitely more disruptive that an entirely new line. Maybe when Toronto has a dense network of subways like New York or Madrid, and Yonge is still overcrowded like today, would it be worth considering. But that won't be in any of our lifetimes.
Unfortunately, there are actually merits to adding an additional 1-2 tracks to the Yonge line. The Yonge subway is the most crowded subway line in North America (yes, even more crowded than the IRT Lexington Avenue Line), largely because our rolling stock is shorter and we're track-limited.
But back to Toronto. Look at the Spadina line, it largely acts like a Western relief line. Guess what? The majority of riders still bus to Yonge. Adding convenient transfers to a relief line along don mills will definitely improve the crowding situation on Line 1, but it won't solve it entirely, especially 50 years out if this city keeps growing and cars become less popular. Not everyone's going along queen, and the Relief line is really a second Bloor-Danforth Line through downtown. It'll be useful for at most half the eastern bus commuters, probably fewer than that. There's just too much density on Yonge for a lot of commuters to justify taking the relief line.
Yeah, remember when we were told that the relief line must go all the way to Sheppard in order to actually work? This point has been completely lost since the debut of the Ontario Line.
Because we're getting some of the line to Eglinton. It'll help, but the majority of traffic that clogs the Yonge line comes from North York. Getting people off of Yonge stations at Lawrence, York Mills, Sheppard is still imperative.
I'm less concerned because getting it to Eglinton was the hard part.
Since the Eglinton to Sheppard section is located in a suburb and not in downtown, it will be easy to get approved, as per the rules of our regional transportation planning framework. This will be quick too because unlike the section between Rosedale and Lawrence, once the denizens of North York realize that they can't board Yonge subway in the morning, there will be public outcry and the political leaders of all major parties will respond immediately with funding for the Sheppard extension.
We just have to pray that the Sheppard extension they fund is for the Ontario Line and not for Line 4.
/mostly sarcasm
Assuming they actually build the ontario line as is, and they eventually want to extend the line to Don Mills station, they'll probably do both at the same time. You can tunnel from one shaft both east and south, saving construction costs.
These people are fiunny. The Ontario Line is being criticized for not going to Sheppard. I'm guessing the Relief Line as was being planned and funded was also going to the Sheppard right.
Contrary to popular belief, the Ontario line has less funding than the Relief Line. At least the Relief Line had funding for studies.
Also, not contrary to popular belief, the line was being planned to Sheppard. The Relief line was supposed to be built in 3 phases. Relief line north would have started immediately after Relief Line South was built. Interestingly, we have a construction worker shortage, and building huge 15km tunnelled lines isn't really feasible right now given how much of a commitment a contractor would need.
Getting the line up to Eglinton is great and all (sort of), but at what cost? How much longer is the relief line South section (the most important part) now going to take? Will we have to spend 2 billion dollars to remodel bloor-yonge because of it? How much more will it cost? Will it prevent other important projects (SOGR, Line 3 replacement, YNSE, Streetcar enhancements, etc)? What are the consequences of building a lower capacity line and not integrating it with the existing TTC system? How will it affect GO? Are there environmental concerns (noise/vibration)? Will the tendering process be further cheapened out and will we be stuck with low-quality value-engineering? etc.
Adding an additional section changes everything, and will likely push back the opening of the relief line by at least 5 years, worsening things elsewhere in the system.