The only activity I've noticed is red "Danger - Do Not Enter" tape plastered over various openings and orange pylons on the platform at the east end. There may be more activity at the west end.
I didn't get around to posting this earlier, but I briefly skulked around the south edge of Union station on both Tuesday and Wednesday on my way in and out of town.
As 41st floor noted, the only real initial sign of construction were single strips of "Danger - Do Not Enter" tape strung systematically across all access points to Platforms 24 & 25, along with polite little signs saying "Closed for Construction" (in maybe 14 pt font on single sheets of AECON-letterheaded 8.5 x 11, very much like it ought to be advising employees not to leave dishes in the office sink). By Wednesday evening, though, the contractors had started to seal up entire doorways at the bottom of staircases, closing them off with particleboard walls.
I imagine the next step will be installing hoarding along the platform edges, which will allow contractors to horse about with a bit more impunity and reduce the risk of workers or building materials winding up on active tracks. As of Wednesday evening, there was no sign of anything having really changed at track level. That might be different now.
In related news, there's been a rollout of a fair number of pretty well-done oblique interior maps of Union Station all over the various public areas, showing platform numbers, access stairs, and so on. They're just printed on ordinary cardstock and look to be relatively temporary, but in light of the next three years of construction, permanent signage would be more than a little bit stupid. In any event, they meet an ongoing need for better wayfinding information quite handsomely.
As of Tuesday the south-end entrance ramp to Union Plaza by way of the east wing of the Telus Tower looked almost ready to go. It's shown as open on the aforementioned maps, FWIW.
Finally, the last of the hoarding has been cleared away from those much-delayed new staircases connecting with the Bay West Teamway. The changes mean that a fairly significant chunk of airy space now becomes part of the current GO concourse.