I personally don't think they need to build the north-side platform at Weston station. As services become more frequent it will become increasingly necessary to separate tracks into express tracks and local, since faster trains would otherwise be unable to overtake the frequent local services. The north pair of tracks makes most sense as the express pair, where trains wouldn't stop at Weston anyway:
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GO seems to be persuing a layout with a express pair and local pair of tracks, rather than a single quad-tracked line with slow tracks either on the inside or the outside. The dual double-tracked setup is less flexible than a unified quad-tracked setup, but it does avoid the need to build a grade separation at Pearson Junction (Wice).
UP Express, despite its name, is the second-slowest service on the line, and Metrolinx has implied that the new stop at Mount Dennis will be in addition to the existing stops, making it even slower. So it makes sense to be grouped with the GO local services running to Bramalea, not the much-faster VIA and GO express services. It and the local service will also be the least likely to experience delays since they never enter the CN segment. This combination is therefore less risky to operate through the at-grade Pearson Junction than if UP shares tracks with the long-distance services.
