Have you thought about how frequent the STC Branch and the Markham Branch need to be. I would guess at worst 10 minute frequency (6 trains per hour) on each, maybe 6 minutes (10 trains) would be better. (Maybe 12 minute SmartTrack overlapped with 12 minute GO RER would work.)
I see no problem with building the Ellesmere to STC, Centennial and Malvern portion. The Ellesmere tunnel would have to be rebuilt. Most assume that the elevated track needs to be replaced, maybe the piers as well. Maybe a cost of about $250M would be needed for the 8km to Malvern. That is $2B.
From Ellesmere to Kennedy, there would be 3 stations, plus 3 minute frequency trains (each way). This can probably still work.
From Kennedy to St. Clair, there are no stations so there is no problem as long as that portion can be double tracked (stacked?)
So all that needs to be worked out is how to handle the LSE corridor when some LSE RER trains (and diesel ones) are added to the mix.
Gweed, you are a smart guy - have you put any effort into solving this problem?
I've given it some thought, yes. The pinch point with this proposal is the Lakeshore East corridor and the Stouffville corridor south of Kennedy. You can widen the Stouffville corridor, but the Lakeshore East corridor will need to be very tightly scheduled in order to work.
For the purposes of this, I'm going to break it down between Red Line (SmartTrack) and Green Line (GO RER) services. In reality though, SmartTrack is just the 416 sub-service of GO RER. The way I see it breaking down is this:
During Peak:
-10 min Green Line service from Mt. Joy
-10 min Red Line service from Unionville
-10 min Green Line service from Malvern or Seaton (depending on if the Province wants to chip in to extend it further)
-10 min Red Line service from STC to downtown via the Central Tunnel
-10 min Special Shuttle service from Malvern to Kennedy
Outside of Peak:
-20 min Green Line service from Mt. Joy
-15 min Red Line service from Unionville
-15 min Green Line service from Malvern or Seaton
-15 min Red Line service from STC
The Special Shuttle during peak is key, because it allows Scarborough riders who are either doing intra-Scarborough trips or are connecting to B-D or the ECLRT to have their own train, to avoid crowding downtown-bound trains. It also means fewer trains are required to use the southern Stouffville and the Lakeshore East corridors.