"That's such a random estimate...based on how long other subway lines took, it'd probably be 6 years. that's long enough that people will move away or permanently switch to other modes of travel."
Oh ye of little faith. If everyone would up and abandon STC altogether just due to a few years of express bus service which will result in a permanent subway link to the area, that tells me STC truly isn't all that important.
"every station would have to be completely torn down and rebuilt from scratch, the tunnel at Ellesmere would have to be widened, probably shutting down GO service for a while, etc., etc., etc."
Why completely torn down? Lawrence East is a very broad station, even if the southbound platform is sacrificed to accomodate the larger subway cars, there'd still be surplus room for people to stand and wait. The n/b platform could remain as is. My only concern would be whether the underground walkway could withstand subways running overhead.
Ellesmere is a toss up because I'd consolidate both it and Midland Stns right at the Midland/Ellesmere intersection. This part would be completely underground hence dwelving under about 100m south of the current station. East of there it'd resume at grade with the addition of Brimley Stn before ascending upto the current STC stn. Further east I'd preserve McCowan as well and hopefully expand to Centennial College and beyond. I'd even compromise Malvern for your Milliken folk if you knew what the word meant :lol .
"And there'd be no rapid transit east of Warden station for pretty much the entire time since the RT would be shut down while the subway was built...other options leave open the possibility of running the RT during construction."
So how exactly did people survive before subways existed in Toronto? I said this before but for emphasis increased service of 9/16 all day, special status for the Stoufville GO line whereby no additional fare is charged to TTC customers to ride so long as they have a valid transfer during the construction period, rerouting of several Kennedy routes to Warden and Don Mills Stns. Again not rocket science, just sacrificing short term benefit for long term gains
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"The general corridor could be converted to subway, but the RT's infrastructure itself? Not a chance."
The corridor's the most important thing, losing the space-age looking plasti-domes I could get over.
"Can the elevated RT supports even handle the weight? Subways full of people must be at least 5 times as heavy as the RTs."
How. many. times. have. I. stated. that. they. would. fortify. the. friggin. elevated. portion. first? Additional beams would work fine and if not they can knock down the EROW and maintain the subway within the same path. My hope's for the former as elevated the line can accomplish more than at grade at a fraction of the cost of underground and remember any expansion north or east would be built in consideration that it's supporting subway cars.