I came upon this map from a recent Metrolinx presentation showing planned improvements to the Halton Sub through Brampton and Georgetown.
The first phase of track improvements includes a new connection from the third platform at Bramalea Station to the mainline, as well as a second track leading westbound out of Georgetown Station/Yard. I would guess that Phase I is coincident with major station construction at Bramalea, which includes a new bus loop, new tunnel and bridge, and parking garage.
A second phase would extend the third track from Mount Pleasant to Georgetown, probably for the benefit of CN freights as it wouldn't connect with the Georgetown GO tracks. Finally, the third phase would build that third track through Brampton Station on the south side, and build a track flyover/flyunder between Mount Pleasant and Georgetown.
There would also be a third track on the GO Weston Sub and another platform at Bramalea for local/RER trains - tying in with the 401/409 tunnel and the plans to finally build the fourth track on the section dominated by UP Express.
As Brampton is now
planning to tear down 8 Nelson Street West and replace the bus terminal (among other things) this will make the third track downtown easier to build.
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