Instead of just building a DRL, is it possible to instead built a RER line down the Richmond Hill line and instead of going to Union it turns towards Queen and makes a DRL and potentially extending it to Dufferin to merge with the Lakeshore West RER line? It would offer all the benefits of a DRL but would make one long route, be far cheaper to build than a total new subway north of Eglinton, and would relieve a lot of pressure from Union.
It's been discussed before, back in 2014 when we were all discussing wtf SmartTrack actually is.
There are a few things that are no-bueno with the Richmond Hill line though.
1) It needs a total rebuild in parts between the West Don Lands and Eglinton and massive upgrade in other parts if we want to upgrade it to RER. The current alignment is prone to flooding, is circuitious, among other difficulties.
2) No interchange with the Bloor-Danforth line, kind of the point of the Relief Line.
3) If we somehow made the interchange work, it would be too west to maximize effectiveness (at this point, you may as well just head to Yonge)
4) Not a great location for interchange with either Eglinton or Sheppard lines either.
5) Destination would be Union station, which is out of the way for most commuters heading to King/Queen area.
6) Several studies have come out saying that GO-RER will not provide relief to the Yonge line.
7) Richmond Hill line has very low ridership today.
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I have proposed in this thread before the idea of abandoning the Richmond Hill line and using the corridor north of Eglinton for the DRL, so we could build the DRL subway at-grade for cheap all the way to Richmond Hill.