Sheppard could be in a trench or at or above grade in places - it also does not need to be 100% tunnelled (who said it did?).
Realistically the grade on Sheppard drops significantly east of Bayview whereby two-thirds of the current line could've been built above ground. The underground construction around the Don River alone accounted for a third of building costs. It's doubtful the TTC would give up the trend it's on now dispite the glaring reality that the rest of proposed line would go through low-density sprawl, highway and railway corridors.
Your point is that Eglinton is better because it doesn't need to be run underground for its entire length, a technical fact I agree with...how does that make Sheppard extensions less viable?
Eglinton's the poor man's Sheppard. While the purple line's condo-centric, hub to hub; Eglinton for the same cost to build Sheppard to STC would most likely cover more kms as cost/km is less, serving more commuters. Hence Don Mills-Brimley would be more like Yonge-Kennedy.
Sheppard is relevent because it's one of the busiest routes in the city (if the subway was removed, it'd be the single busiest surface route). Sheppard would take people across the city and there'd be a half dozen major nodes along the way. Eglinton is simply not superior to other potential routes for "cross-city and inter-regional" trips.
I apologize but you're wrong about this. Sheppard built to full-length would still barely get someone a third of the way across Toronto. It also would never be inter-regional as it wouldn't directly interface with any suburban routes beyond the ones that ALREADY exist at Don Mills (Agincourt GO would need serious revitalization and more than 6 peak trips a day to be of use). About it becoming the busiest surafce route, that's debatable because alot of the 40, 000 people who use it daily only do so because there's
no other option. Hence most of these would likely revert to the original routes they'd commute on prior to Sheppard's construction if it were axed. Eglinton on the other hand, has so many patterns of use you'd never see a half-emply train on its line off-peak. Consider this...
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Kennedy, Eglinton, Eglinton West interchanges
-13 stop bell-curve to Bloor-Yonge bypassed
-Real subway east of Kennedy (Shep's only caters to STC; while continuing onto the Kingston Rd corridor would link Guildwood GO/VIA Stn, Cedarbrae, the Morningside/West Hill area, U of T Scarbourough and inter-regional access with Durham via Hwy 2).
-intensification of Don Mills. Possible S-bahn into Thorncliffe Park area diminishing the need for a full-length Don Mills subway in the future
-BD alleviation, (some YUS alleviation as well)
-increases usage of the Spadina Line as commuters now have viable link between it and Yonge north of Bloor (Shep's has zero density intermediately to support a subway)
-Triggers development of Eglinton Flats, Mt. Dennis, Weston area
-Brings subway transit to Mississauga/GTA, airport/ACC and Hwy 27 corridor
I'm not saying never, I am saying in the long run which line benefits the greatest number of people (there's some utilitarianistic philosophy for you!)?
Is a Senlac stop more important than a DRL? Of course a full DRL or a very long Eglinton line would be better than a single 4km Sheppard extension, but it's a silly comparison - having one does not prevent the other.
It's does when gov'ts can only afford to build one at a time. It could be several decades before another chance for Eglinton or DRL comes by. Why screw it up now, by overtly inflating the significance of Sheppard, a line with limited potential when there are two more important gaping holes on the transit map commuters are begging for to be filled in?