I wouldn't focus entirely on relief, a 1980s idea for a downtown line emerging before we saw the revitalization and increased density of many east and west end neighbourhoods between downtown and the post-World War II suburbs. We can bring rapid transit to numerous neighbourhoods in the urban east and west ends in Toronto between downtown and Etobicoke/Scarborough where people want to live, work, and spend their leisure time while achieving relief for Yonge.
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I understand that a new subway line in the east end is most urgent to address capacity issues on Yonge, but the situation in the west end also merits a branch shortly afterwards if not at the same time.
Well, we're talking about two distinct justifications for building a new downtown/Old Toronto line:
(1) to bring new rapid transit to dense urban neighbourhoods
(2) to relieve a bottleneck on the existing rapid transit network, thus allowing the Yonge line to be extended even farther into the suburbs
I completely agree with you that #1 has great merit, but the unfortunate reality is that nobody outside of Old Toronto cares about it, and #2 appears to be the only politically realistic justification for a new downtown subway. (And this is not just a Ford-era thing -- the political climate in Toronto has favoured suburban subways for a long time; maybe this can be changed, but good luck...)
Given that reality, I don't expect to see a DRL West until the University-Spadina line is just as packed as the Yonge line currently is, and despite being busy in rush hour, it still has quite a way to go. So if there's an alternate proposal to bring rapid transit to this area, it probably warrants consideration.
Even with the Nunziata/Layton version, the ARL at most can provide a relatively limited collection of stations in the isolated railway corridor onto which many neighbourhoods turn their backs. It would be aimed at people travelling to Union or the airport
This I don't agree with -- the stop spacing between Bloor and Lawrence shown in
this map of the Nunziata/Layton proposal (Weston Station should be shown at Weston & Lawrence; the map is wrong) is equivalent to subway stop spacing, so it's not really a "limited collection of stations." And we can't realistically expect a DRL West to go as far north as Lawrence (or even north of Bloor), so the Nunziata/Layton proposal would bring rapid transit to a much larger area than a DRL subway. Also, the railway corridor is not
that isolated -- the stations in the Nunziata/Layton proposal would intercept many surface routes (Lawrence, Weston, Jane, Eglinton, Rogers, St. Clair, Dupont), providing a huge increase in mobility for people using or connecting to these routes, regardless or whether they're going to Union or the airport.
South of Bloor there are fewer benefits, but south of Bloor we also have easy access to the Bloor subway and streetcars that go straight downtown (and even DRL subway proposals usually only add one extra station between King/Queen and Bloor -- at Sorauren or Howard Park).