"Trains returning to Vancouver would leave Union Station either from the East as it came in from (if turned around prior), or from the West. In the latter case, the train would then proceed North along the Newmarket Subdivision, shared by GO Transit's Barrie line. This subdivision is a portion of a former route for the transcontinental line; however, the train only continues as far as Snyder Diamond in Vaughan. There it would then proceed Eastbound along the York Subdivision towards Thornhill to meet up with the Bala Subdivision at Doncaster Diamond and from there continue Northwest leaving the Greater Toronto Area towards Washago and eventually Vancouver."
That was my guess, but that 'diamond' in Vaughan (actually an overpass) doesn't allow a northbound train to switch directly eastbound. They'd have to pass that diamond, back themselves down the lead onto the York sub, then continue eastbound:
Which of course must be exactly what they're doing, but man, what a painful move for a passenger train...