Lakeshore East doesn't have a train depot, though it is getting one. Lakeshore Wests yard also holds the trains for several of the other lines.
My major concern with 15 minute costs is the cost of grade separations, with hourly service it is easy enough to justify not doing them but with 15 minute service they are needed much more badly on many streets.
Going through the lines, a brief look at satellite imagery nets me this amount of at grade crossings to eliminate if you were to run the 15 minute service to the previously planned ends of AD2W service:
Stouffville to Mount Joy: 14 separations, 1 at grade crossing and 4 closures
Richmond Hill to, well, Richmond Hill: 3 separations, 1 closure, 1 junction separation
Barrie to East Gwillimbury: 17 separations, 2 closures, 2 remaining at grade that can't be closed but aren't really worth grade separating, and 1 junction separation.
Kitchener to Mount Pleasant: 2 to get it to Mount Pleasant, and additional 21 separations, 13 closures, and 4 that shouldn't close but aren't worth separating. (Most of these are just farm driveways) to get it to Kitchener.
Milton to Lisgar: 6 separations, 2 closures, 3 that aren't really worth it but can't / shouldn't be closed.