Beats me. But green roofs still require occasional maintenance (humans to walk on them). Perhaps their worry was irrational. I don't know.
Nontheless, I would have preferred victorian-glasshouse furrows instead of a flat opaque roof. Better views of CN tower and saves on lighting too. And likely safer during the day with sunlit illumination -- train drivers having difficulty adjusting quickly to dark -- when driving train from sunny outdoors driving train into currently-dark shed -- and sometimes having difficulty seeing crowd-spill in the dark after being sun-blinded.
No green garden. No glass. I look forward to Union revitalization's endgame except for... the utilitarian shed. A missed opportunity indeed.