The capacity of Stouffville 404 station would be whatever they arbitrarily decide to make it considering it is surrounded by open land and there is no limit to the size of the parking structure they create. Spreading growth from one station to two stations in the greenbelt, to avoid the cost of building upwards, would be sprawl which is exactly what the legislation is meant to prevent.
Whoa, man, I can't agree with this at all.
Yes, the current model of oceanic asphalt GO parking lots is not hunky dory from an environmental or urban planning standpoint. The model needs to change, particularly in the more dense locations.
But lets get a sense of scale, here. Totalling up GO parking lots as if they're making a significant dent in the total volume of arable land in the Greenbelt is lunacy. You could quadruple the paved footprint of every GO parking lot on the system and you'd still probably eat up less arable land than a typical new golf course. Since /Places to Grow/, every GO EA taking place in Greenbelt lands has been obliged to consider agricultural usefulness of greenfield parcels no differently than it might look at the natural heritage value of woodlots or riparian areas, and tilt the site selection accordingly. That's why the Baden station on the Kitchener extension---which pretty had to be greenfield---is nonetheless going to be built on a former fertilizer storage site, IIRC. Considering the report mentions how the Stouffville Road (Gormley) site is mostly owned by the City of Richmond Hill, it sounds like they probably not eating into prime farmland there, either.
It would sheer financial insanity to spend money on a Burlington/Pickering-style parking structure at Gormley, if and when they max out the surface parking capacity on the station's parcel of land. You could get two new stations with surface parking closer to commuter driveways at a similar cost.
Not only that, but I'd seriously question whether paving 5 more acres would be a greater evil environmentally than building up... you're talking about diverting ~$100m from other transit service improvements and pumping who knows how many tonnes of CO2 to make the necessary thousands of tonnes of concrete while not removing a single vehicle-kilometre from the 404.
In any event, this might be rather moot as I wonder if there're really all that many additional passengers to gather as you push further north alongside the 404 beyond Gormley. Aurora, Newmarket, Sharon and so on ought to be adequately served by Barrie line trains through the existing stations. And if they build the layover at Bethesda Road, a Bloomington Road station would come with an operational handicap, as trains would have to deadhead northbound in the morning before reversing for the run south.