--- Long post ahead ---
Since we are talking about a Dufferin alignment I made a map to visualize it and while it has merits I think it also has issues. Firstly is the cost as stated in my last post since the overwhelming majority of this extension would need to be built underground. Secondly I have to wonder how this would affect Line 1 north of Eglinton since the two routes would only be about 800m apart. I have to wonder how many new riders would be generated compared to just pulling preexisting riders off of Line 1 especially since the Spadina side of Line 1 still has capacity to spare compared to the Yonge side. For comparison Yonge and Pape are 3km apart so it not only relieves congestion on Line 1 by intercepting riders earlier, but also can generate new riders in a area that is distant from the closest north-south subway line. By comparison north of Eglinton what does the OL offer that Line 1 already doesn't? South of Eglinton things are different and there may be a business case but north of it I just can't see a Dufferin alignment being all that useful. It's why I think a north-south line in the west end will more then likely be on a street like Jane since its farther from Line 1, meaning it will not only intercept east-west travellers sooner and shorten their commutes, but it will also provide a north-south line in an area that doesn't have one thus generating more potential new riders. I think the time savings to be found here would be negligible since the people already on the Dufferin corridor are already within a couple minute bus ride from the subway, and for people commuting from further west that negligability only increase. For an example a person coming form say the Wilson & Weston area may save a small number of minutes transferring onto the OL at Dufferin, but that time saving could be magnified if they transferred at Jane instead. North of Wilson the business case for this alignment becomes a bit better but the main issue is strictly between Eglinton and Wilson where we have two north-south subway lines in an area that doesn't have the density or ridership to justify them that close togeather. If we are going to build another north-south line in the city's west end then at this moment a line further west like on Jane would be the far better and more productive option in the long term. Something does need to be done in the future on Dufferin, however Jane Street just presents itself as the better immediate option since it is in a nice middle area equal distant from Line 1 and the city's western boundary so it would not only intercept east-west riders sooner then a Dufferin alignment thus lowering commute times, but it would also generate more new riders as well since it is in an area with no north-south rapid transit options.
With all of that said for this alignment I did what I usually do (800m-1km stops spacing). I included a stop at the Downsview redevelopment although it doesn't have a name since that area doesn't exist yet. I was also considering a stop at Cartwright Avenue on the west side of Yorkdale Mall since you could argue the mall could support 2 stations, however this is where I also began to think about the issue I addressed above. Finally the stop at Finch is a troubling one since you obviously want the connection to the 36 and an eastern extension of Line 6, however the community of Bathurst Manor would be far better served by a stop at Overbrook Place so do you build both or do you build only 1? If we put a stop at Finch then Line 6 has to be extended east to Dufferin which would not be hard at all since there is more then enough space for road widening on Finch between Keele and Dufferin. Basically this alignment is great south of Eglinton and questionably north of it, so perhaps a line along Dufferin would be better built as a stand alone line running between the Exhibition and Eglinton Avenue, or perhaps it could go all the way to Steeles but anything north of Eglinton would be unnecessary until both the Spadina section of Line 1 and a line on Jane are at capacity or the density along Dufferin north of Eglinton dramatically increases. Another option would simply be better service and a couple more stops on the Barrie GO line but that of course would rely on Metrolinx getting their heads out of their asses. Of course there is also the ultimate hail marry, this will never happen proposal and that is rebuild the Belt Line, but that's as realistic as Metrolinx figuring out how to run a modern electrified regional rail network.
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Geographic map:
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Third map a tossed together, this one is more fantasy but its just to show that Dufferin could be served by its own line while the OL goes to Jane. I included 2 possible northern terminus and extension alignments. The first is terminating the line at Fairbank which would leave open the possibility of a further extension north on Dufferin. However it wouldn't be the most convenient transfer for those who want to use Line 1 since they would need to transfer onto Line 5 first. Option 2 puts the northern terminus at Cedarvale and could allow a possible northern extension up Bathurst which has far more residential development although this may be more technically challenging. The upside is it would offer a better transfer to both Line 1 and 5 simultaneously. Both Dufferin and Bathurst were considered alternate alignments for the Spadina extension back in the 60's but were rejected due to cost so running a subway along Bathurst or Dufferin isn't without precedent its just the current Line 1 right in the middle makes justifying either Dufferin or Bathurst north of Eglinton hard.
Personally I could see Bathurst in this case being the better option. Density along Dufferin begins to drop off north of Lawrence as its replaced by industrial land all the way up to Steeles. A pro is that it would hit the Downsview redevlopment and would have a really nice transfer with Lines 1 and 4 at Sheppard West as well as a easy transfer with Line 6 at Finch (only a 2km extension needed). On the other hand Bathurst is pure residential all the way up to Steeles so its density doesn't drop off at all and argugably increases as you get closer to Steeles and this residential development continues beyond into Thornhill offering for a far better extension into Vaughan. A bathurst alignment would still maintain a connection with Line 4 at Sheppard and a possible connection with Line 6 at Finch (now needing a 4km extension). That said another downside would be that Bathurst could end up being a NIMBY nightmare since it is one of the wealthiest corridors in the city. However it could be argued that Dufferin becomes the better alignment north of steeles since it is a perfect 4km in between both branches of Line 1 so it would be a better interceptor up there, while Bathurst is only about 2km from Yonge Street. Basically there is no simple solution to this that checks every box. I think the simplest option may just be better GO service on the Barrie Line with new stops at Wilson, Lawrence, Eglinton (under construction), St.Clair West, Bloor (already propoed), and Queen Street (already proposed) and a 15 minute electrfied service.
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