It would be interesting reimagining the DRL as a relief line or distributor for a notional GO RER network. DRL could become an upside down U of sorts, intercepting passengers at a Cherry/EDL station and at a Spadina/FortYork station, giving an alternative route to the north end of the CBD.
What I favour is building the DRL as part of the GO REX system. Have 4 lines: Lakeshore (E+W), Milton + Richmond Hill, Kitchener + Markham (Kitchener to Mt. Joy), and Pearson to Seaton via STC. All of those routes would run via Union.
The first 3 would have second, shorter routes that, rather than going through Union, would go through a DRL tunnel under Wellington, King, or Queen. The routes would be: Clarkson to Pickering, Mississauga Centre to Richmond Hill Centre, and Mt. Pleasant to Unionville. 15 min frequencies on each of those branches equates to 5 mins off-peak through the core, with shorter headways during peak.
The Richmond Hill branch, rather than running through the Don Valley, would run the traditional northeast DRL route to Eglinton, and then continue under Don Mills until just north of Lawrence, where it would then pick up the current GO alignment up to RHC, with the full GO REX route continuing to Bloomington. With this, you get 7.5 min frequencies up to RHC outside of peak, and potentially as long as every 3 mins during peak.
This solves the capacity crunch at Union. It solves the overcrowding of the Yonge line, because in essence the DRL is reaching all the way up to RHC. It alleviates the subway system in general, because not only does it provide a more convenient option for 905ers who currently get dropped off at Finch or Kipling, but it also provides alternatives for 416ers, because within the 416 and inner 905 these routes are running near subway level frequencies.
