Ok, I've found some pictures on the Metrolinx Flickr Feed as Well as the Tumbler Account of a one
jacoblorinc, so you can see not only what is happening above Union Station but deep below and underground, under the station and tracks. They are building a retail concourse to relieve some of the crowding that occurs in the station at present. It's a huge station, but the concourse used for GO is rammed to capacity at most times, and especially during the rush hours from 3pm to 7 or 8pm it's difficult to get through the station, and sometimes even later when there is a game at the ACC the station can be clogged until 11pm which is pretty close to when operations shut down.

Courtesy
Metrolinx
And Here take a look what is happening below ground
jacoblorinc on tumblr
This new area being dug from under the existing tracks and station will form the new concourse area with retail. It will allow for a massive increase in passenger capacity. Which of course is greatly needed due to massive overcrowding in the current system.
Here is the existing Go Concourse which will now sit above the new concourse which is being constructed deep underground. The current capacity of Union Station including the existing concourse is believed to be around 300,000 passengers a day. With the renovation that presently underway, the capacity of the Union Station Terminal will double to over 600,000 passengers per day. The scale of the project is impressive, and the time to construct it is also monumental, as it will likely not be completely finished until 2018 or at least 4 years from now.

from
bobolink at
flickr.com
This project, along with the Airport Rail Link, and the Eglinton Crosstown LRT (Underground Portion Runs Through Old City's of Toronto, York, and East York) are the most fascinating Toronto Transportation projects, and while there are others, such as the Spadina Subway Extention, New LRT vehicles, New Subways, the scale of these construction projects is impressive, and of course badly needed do the the capacity issues transit in Toronto is currently experiencing.