I was just perusing some pages on GO's website and I noticed something I hadn't seen before:
http://www.gotransit.com/public/en/improve/projects/kingcity/
At King City GO station on the Barrie line, they are building a second track and platform for AD2W service, and will build "
pedestrian bridges so you can access the new track and platform". The other Barrie Line GO stations where construction has been announced so far (Rutherford, Maple, and Aurora) are having underground pedestrian tunnels installed. I thought this was an interesting tidbit--GO isn't doing so many over-track pedestrian bridges these days.