There doesn't seem to be room for ALTO on top of everything that's already there. Yet stations are called "Union" because they bring everything together in one place, instead of various lines going to different stations. Booting VIA, whose lines are supposed to be feeders to some extent, as are GO's, makes little sense. So something may have to go in a Tunnel somewhere near Union. I would suggest having one end of that station connected to Union and the other to another building (DPB or the Convention Centre to be replaced) could do it.
Whatever goes in the tunnel has to be electrified, so it's either GO's future electrified lines or ALTO. Over a decade ago planners came out with a concept for four-track two-platform tunnel directly beneath Union for Lakeshore GO and perhaps another pair of lines. Another idea was to reroute some of the lines from the Northwest into a tunnel under Front, ending around York. I would say taking some of GO traffic out of the elevated platforms to make room for ALTO is more obvious, but it could be the other way around.
The other possibility is putting ALTO elsewhere, such as East Harbour. That has connections to GO and TTC, and is near the core, which is where ALTO has clearly said they are headed. I think a line station on the lakeshore ROW is nearly impossible given the layout. What could be built more easily is an underground terminus with tracks northbound in a tunnel under the DVP. But that would put ALTO and VIA in different locations, which is Union-challenged. So even though it will be difficult to put something in the crowded area between John and Church, I think it needs to be done.
