One of those tracks is for the Newmarket Sub to Barrie. The current junction between the Barrie and Brampton lines at Parkdale will be withdrawn from service before long.
A reliable source confirmed to me that the new junction at "Woodbine" (I have also been told that it may be named Wice instead) where the Pearson spur leaves the Weston Sub will go into service (partly, anyways) this weekend. The new build appears to be three tracks from Strachan to that point, and two tracks from there to Bramalea. For now, the third track west from Woodbine/Wice to the next new Interlocking ("Airway") is apparently a service track for freight trains, rather than a main line track.
Three tracks gives UPX dedicated service in one direction, and GO dedicated service in one direction. If GO becomes bidirectional, it will be necessary for GO and UPX trains to intermingle in the other direction. It's feasible, but even with all the shorter blocks, it will be interesting to see if the heavier GO trains, making more stops and likely not running to the minute, can coexist with the UPX trains running on 15 minute headways. Or, UPX could have dedicated use of two tracks and GO could run on a headway that puts all meets west of Woodbine/Wice. It will be interesting to see how VIA is handled as well.
I'm also told that the station trackage at Pearson has a subway-style 'X' crossover configuration rather than the more railway style '\ /' crossovers.
- Paul