The current station is not that old, and was likely built where it is with the assurance that GO was going no further ( it is right in the way of the GO line, which dead ends there). So if GO intends to expand, it's likely that GO would have to build VIA a new station at GO's expense. Making the new station a joint facility just makes good use of money that GO will have to spend anyways, And maybe VIA threw in some federal money, so it's a win for GO.
I hope that GO will use this opportunity to pursue the 'town square' type of development for the station, rather than replicating the "boring building in a barren wasteland of parking lots' style of station design.
- Paul