Well, this thread is kind of going off topic with all this Yonge-talk, but as my final comment on the matter I think talk of Yonge line overcrowding is a bit overdrawn. Even with a Yonge extension and without a DRL the line's ok out to 2031. Between fold up seats (or even just all longitudinal seating) and a 7th car we should have some cushion for like a decade.
Plus, it's important to remember that Yonge's congestion is, mostly, PEAK congestion. Kind of an obvious point, but if you ride the Yonge line at 9am it's hardly swamped. Building a huge new twined subway would really only benefit the few hundred people who have to wait 3 or 5 minutes for the next train.
I think any DRL would give the Yonge sufficient breathing room for a few decades. Beyond that, the City will have to look at shaping demand to discourage our over-reliance on Yonge. It's been a conscious decision to funnel almost all of our major feeder routes onto Yonge and locate soo much development along the corridor (while also discouraging intensification elsewhere like Dundas West.
Do you ever ride the Yonge line? Allot of the time people can't even get on trains. And from what I've observed, a 7th car won't solve the issue.