Well, the challenge of building in a space that you may not completely own, while accommodating those who own the air rights above your planned station, with plans to place a complicated set of structures over top of your platforms, with some constraints and certainties about how it would connect to things like PATH and transit and the street grid.....
Having said that, it's not a bad idea. An anecdote - I recently took a train from Brussels Central Station to Amsterdam. Using my North American assumptions, I turned up on the platform 25 minutes before departure. My train was not there.... in fact, it was the sixth train to use that platform in those 25 minutes.
The Spadina-Union opportunity strikes me as very similar to a Brussels Midi -Bussels Centrale pairing. Once we get further along an operational mindset where trains come and go that freely, having a stop a stones throw from Union is quite doable. But I suspect the situation will have to get a lot worse before anyone puts it forward with conviction. Hopefully the development over top will not happen first, in a way that precludes it happening later. Timing is everything.
- Paul