UPX shares track 1 at Union that is used for Platform 3 for VIA Rail. It share all GO/Metrolinx tracks in the main corridor.
It was its own tracks from the Weston Sub to Pearson that is 100% elevated.
Neither Union or Pearson Station can be expanded for longer trains. What good expanding the other station when the main ones Can't?? To expand Bloor and Weston, you have to close a whole platform and force riders to used the other to get to the GO section.
Easy to do for the centre platform, but becomes an operational problem when rebuilding it. You will only be able to use a single track at both stations for UPX for the other platform while the centre one being rebuilt. This will require longer headway than the current 15 minutes. Does effect GO since they will be slow order passing the construction for the platforms. There is only enough room at the 2 station for one more car and possible at Pearson. Not so for Union.
Yes you will get some riders who will walk that last mile from Union, but they are the exception, not the norm.
If you need more room for riders, you use double deck cars in place of the current fleet as 3 car sets.
I have photos and videos of the Pearson Spur as noted in my handle. Tag: UPX, Union Pearson Express and they will show you what it looks like.