Diminutive, your reconceptualization of the GO system as an RER style network is fascinating and something that absolutely must be developed in the coming decades.So this is another iteration of what I've been trying to promote here in terms of wrapping the DRL together with S-bahn or regional style rail.
There's nothing especially unique about any particular component of this, but cumulatively it would represent a major improvement to transit in every part of the city; improved transit in downtown's 'shoulder areas,' rapid transit access for the extremities in Etobicoke and Scarborough, genuine rapid transit for the 905 and substantial relief for the Yonge Line.
The basic idea would be to create four major radial routes (Lakeshore West, Weston, DonMills/DVP, Lakeshore East) which would connect to a central distributor line through the core (the DRL).
This route is more station dense than some DRL versions, roughly approximating the Bloor line. Other major changes would be the Front->Queen kink along John, and the interlining.
The kink would basically allow more stations downtown, one @ MTCC and one in the Entertainment District. Both of those areas have seen proposals for quite large redevelopment (Mirvish Gehry, the Oxford-Casino-concept) and have seen substantial development already (e.g. 'Westcore'). One issue most DRL proposals have is lack of CBD stations (e.g. Osgoode/Queen, St.Andrew/King, Union). Fewer stations = fewer destinations, which leaves Yonge as a more attractive route, reducing any relief value.
As for the interlining, it would enable more branching in the suburbs. The lines would join together between Gerrard Square and Dufferin/Queen. It would have to be designed to handle very high train frequencies (90s would be ideal) so that branch service would still be fairly quick.
From Gerrard Square, one branch would turn north along Pape to Eglinton-Don Mills. In my route, the railway would avoid Flemingdon Park and just cut diagonally through Thorncliffe (at-grade) to save money, running time and allow a better interchange with the Crosstown.
This branch would then break into two smaller branches, one travelling along DonMills/DVP on an elevated viaduct to Buttonville and the other along the rail corridor to Agincourt and STC. The connection to STC would obviate the need for any kind of SRT replacement. Riders would instead get a quicker, one seat ride downtown.
I also included a branch along the rail corridor into Malvern, but it would likely be better to scrap that and increase frequency on the other two branches. As you can see, nearly every station is intersected by two or more high volume surface routes. Not shown on the map, but I imagine the DonMills/DVP brach extending to the proposed Buttonville redevelopment. It's apparently one of the largest single development projects in the region, and the 404/407 area is already one of the largest employment areas, so it deserves some service.
The second major branch, which is more strait forward, would be a rapid transit version of Lakeshore East. Add a few stations here and there. This service would run to either Pickering or Ajax to serve the 905 commuter market better.
Notional AM peak schedule (no Malvern branch) would be: 10 trains to Pickering/Ajax, 15 to Buttonville and 15 to STC.
Similar to in the East. One major branch which would run in the Georgetown corridor, recycling UPE's track space. This branch would continue onto Bramalea or Brampton, depending on ease of construction. One branch would break off after Weston station to run, through an elevated viaduct on Albion, to a new transit hub at the Albion Center. Like in the East, most of these stations intersect several high volume surface routes, particularly the Albion hub. This would save people having to make long bus rides to the Spadina or Bloor Lines.
The UPE spur and station would be reused as well. If the soon-to-be-new Pearson station can't accommodate longer trains, it may be easier to just have an entirely separate people mover shuttle line between Pearson and the corridor. In the long run, it may also be worthwhile to extend the spur, around the airport, to the office cluster on Matheson. Apparently it's one of the region's biggest...
The other major branch would again be pretty strait forward rapid transit version of Lakeshore West. From Dufferin/Queen, the line would continue underground to Sunnyside then stay in the railcorridor until Oakville or so. In Toronto, I'd place new stations at Queen/Lansdowne, Sunnyside, Parklawn & Kipling.
Notional peak schedule: 4 trains/hr to Pearson, 10 to Oakville, 12 to AlbionCenter/Rexdale, 12 to Brampton.
GO would need to be completely rethought. My proposal would cannibalize most of GO's busiest markets (Lakeshore West, Oakville->Union; Lakeshore East, Pickering->Union; Georgetown, Bramalea->Union).
The TTC's surface network downtown would need to get rethought as well. People like Steve Munro continually insist the DRL and the 501/504 serve totally different markets but I don't believe that. Maybe the streetcars wont get completely cannibalized, but surely travel demands will change substantially. Quite detailed modelling would be necessary to figure out how exactly the surface network would need to change.
That said, what happens to the regular Lakeshore railway tracks going through Union? Are these replaced by the underground DRL along Queen-Front?
Also, the DRL routing you have proposed doesn't appear to be very effective at diverting traffic away from Union, as passengers wishing to travel on the YUS line have no choice but to transfer at Union. Would it not be best to keep the Weston-Don Mills tracks along Queen and leave the Lakeshore lines to travel through Union?