Can I mention an alternative DRL routing I don't think I've seen suggested here?
I think it should run above ground on GO's Don Branch, terminating in the north at Broadview station (requiring a tunnel of about 500m), and leaving the Don Valley to enter a 2 km tunnel under King St., terminating at the existing King Station.
That seems to hit the goal of relieving Bloor-Yonge, not interfering with Union and the USRC, and improving transit in downtown east. Its chief advantage of course is that about half the route and a couple of stations would be above ground, and it is shorter than proposals based on Pape. So it would be a lot cheaper.
Personally, I don't think a DRL will ever get built unless it is cheap, which means above ground.
I think this would have to be run by Metrolinx cause they wouldn't give up a line, even one they are probably never going to use. (If you think they would use the Don Branch for Richmond Hill trains, have an up close look at the Half Mile Bridge someday.) If they wanted to extend this line west to a station at their Bathurst Yard I guess they could too.
I'd be glad to hear why this is technically or politically impossible, or otherwise a dumb idea. I think it might never have been considered much, because in the old days of the DRL the Don Branch wasn't owned by the government, and the Lakeshore corridor wasn't so busy.
(I think that in 300 posts on UT this is my first "fantasy map"!)
I'm assuming you mean the tracks on the west side of the river, right? Because north of Gerrard, a set of tracks crosses over to the east side of the river. That may be the better option, because there's more room on the east side north of Gerrard than there is on the west side.
It certainly would be a cheaper routing, but it would mean that you would have stations at Gerrard and Queen on the northbound stretch, and that's about it.
Definitely an idea worth examining further though!