I have a question; is this park actually even possible? There is currently no electrification whatsoever along these rail lines, and as far as I know, the ventilation requirements for these diesel electric trains would make it cost prohibitive, or even impractical to put a park on top. Sure, GO RER involves electrification, but that's going to take a long time to actually happen, and even when it does, as far as I know, it will not include all lines in one phase. Further, the retention of the old train shed support columns at Union as a "heritage" item, prohibits the installation of a catenary electric system.
I really like the idea of a park here, and hiding the train lines, but considering the above, as well as the staggering cost, which could easily be spent on another subway line, I'm a little hesitant. Perhaps a private partnership, like the ORCA Project, while not ideal, would be a better use of resources? Even Chicago's Millenium Park only cost $500m, this just doesn't seem reasonable at all, to me.