Why would one want involve themselves with the hassle and expense of building atop a railway corridor if they could avoid that. I'd think it would be cheaper to rebuild the Gardiner entirely and avoiding the mistakes of the previous design.
AoD
Well you would be rebuilding the Gardiner entirely, or at least the eastern portion of it. Putting it on top of the rail corridor would allow Lake Shore to be reconfigured and have it out from under the Gardiner's shadow (literally). The rail corridor is already an eyesore and a barrier to the waterfront, so might as well stack two barriers on top of eachother.
Now having said that, my preference is still to dig a trench along the central waterfront, put a stacked tunnel in it, and then put DRL fill on top of it to create a linear waterfront park across the entire central waterfront. That would open Gardiner lands up for redevelopment, remove an eyesore, and Toronto would get it's version of Chicago's waterfront.