I don't know... part of the tunnel is already built under city hall, and it connects with an operational tunnel under the tracks. I don't see how it could be any more difficult for us to build a tunnel under a street if Vancouver could build one under false creek? We have underground parkades in Calgary only one or two hundred meters from the river... so I can't really see that being part of the issue. The tunnel would be under 8th Avenue, about a kilometer from the river.
PS. I agree with you about Grandin Station being too deep, that's where my bf and I got off the train the other day to start exploring downtown, and it's quite the trek just to get out of the station, especially considering his legs were in serious pain from rugby