The Toronto Star ran a story earlier this week about how a Downtown Relief Line would make life easier for the suburbs by relieving congestion on the main lines. This is not incorrect. The following image from the story however...
The main problem with it is the assumption that the line should run to Union. This would be a categorical mistake because we already know Union will approach capacity around the middle of this century and we need to divert people from there. Not only that, but where's the incentive to users to get off the Bloor line to goto Union to get onto the Yonge Line and backtrack north along one arch or the other? There really isn't one, so the argument that we're going to free up space at Bloor/Yonge this way is flawed.
Because of this, any Downtown Relief Line needs to cross through at Queen so that you both relieve traffic at Union and allow people to cut off some subway stops if they're heading mid-downtown. In fact, an older Toronto Star story about the capacity issues building up shows an Metrolinx projected route that does just that:
My primary problem with this later plan is the swing south to Exhibition Place which seems unnecessary given the seasonal use nature of the place at the moment. I'd rather see the west end run straight through to Dufferin/Queen, link up with a new Go Hub there, and divert people from Union at that location before eventually extending through into Etobicoke. With the Streetcar right-of-ways currently in place to service Exhibition Place likely capable of handling the traffic, we could then look to a north/south midtown line to run there in the future should the nature of use for the location change.
Likewise, traffic could be further relieved upon the Yonge line from the East by making use of the Don Mills rail right of way to build out a surface transit line that would run down to the Port Lands as envisioned in the first Toronto Star graphic and could serve to move people between the Bloor, Sheppard, and proposed DRL.