Do you know the joke that says Toronto has 2 seasons? Construction and Winter?
Well, I've been at Union Station different occasions during the last 3 years, and it has always been under construction -straight to my nerves-. If the subway is delayed, if the stairs took you through confusing tunnels, signals, and frustration, don't worry, it hasn't changed yet (be calm an give yourself a -what if 30 min- before your departure time).
At least this time we had enough time to reach the Tim Horton's, get a coffee, scratch our heads and look at each other when trying to decipher the map to reach via rail...
Trains depart on time, don't think they will wait for you. Better to be on time making the line.
PS. You'll need enough TIME.