Lots of interesting points being made. Here are just a few comments on a few of what I think are the biggest issues surrounding the Gardiner question.
1. The Viaduct Option.
What a stupid idea. Maybe in some peoples minds it looks cool and the idea of building over the rail tracks might be a possibility. But a land bridge? If ever there was a case of wasted money. I dont see how anyone can actually take this proposal seriously other than too maybe spark the imagination into being more creative on realistic options.
2. Context
The Gardiner should not be viewed simply from a transportation standpoint or as a single entity. As many have mentioned seemingly forgotten aspects such as onramps will make a huge difference and take up their share of room. The Gardiner also no longer runs through a benign industrial area, but is being surrounding by development at a rather rapid pace. Taking down the Gardiner is no longer a case of connecting the city with 'theoretical' places that may come to be. It would connect actual places that have been, are, and will continue to be built. What will the space look like in 30 years? Probably quite a bit different and this needs to be kept in mind so that in another few decades people are not sitting around wondering how to fix the problems that the second reincarnation of the Gardiner has brought.
As I mentioned before, from a transportatin perspective, this corridor should not just be considered from another highway. Rail is hugely important to the downtown core, and will likely find this importance grow dramatically in the years to come.
If you look at Toronto and the GTA at large, probably the most important transportation project that will be underaken in the next decade is a proper upgrade of the Lakeshore line. And given that the main terminus of the line is at Union Station and that a major overhaul will have to take place on the line especially within a few kilometers of the station, the Gardiner corridor will be very important. Maybe sections of it could be used for new passenger only track? Maybe for temporarily rerouting some trains? Diverting traffic. Etc. Given the rail corridor and the Garinder are so close neither one can be examined without taking into account the other.
3. Cost.
How to pay for it? I see no reason why tolls should not be started on the Gardiner tommorow if it was decided to replace it with a tunnel. They dont have to be excessive tolls, but something that can help offset construction costs. If the Gardiner is simply going to be replaced with surface routes, then tolls might be a more difficult proposition.
The entire project (especially if it were to include rail upgrades) would be massive. Funding is going to have to come not just from traditional government sources, but private sources as well. Maybe the city can sell off land holdings? If rail upgrades were part of the plan, CN could be encouraged to join the project. However its funded, in what percentage, by whom, its going to require a bit of creative thinking that might make some a little uncomfortable by images of P3's and privatization.
4. Do it right, or dont do it all.
Leaving the Garinder as is might not be ideal, but it could be a lot worse. The problem is once you take down the Garinder, you open up a whole new set of problems which will need to be adressed. Burying the Gardiner is not a magical solution that will fix up the waterfront with a wave of a wand. Its a part of a number of investments that are taking place that all put together over several decades will ultimately lead to the vision that people have.
Id rather see it stay and let a compentent group of citizens, government officials, planners, and developers tackle the problem then have a bunch of nitwits who either leave the city with a project that cost 10 times the original price (Boston), or, leave the city with a problem it will only have to fix 10 or 20 years down the road (The Garinder itself).