They may run at an inconvenient time, but the 5:02 pm and 6:02 pm GO trains from Weston station run all the way to Kitchener. There are likewise two inbound trains from Kitchener each morning that pass through Weston on their way to Union Station. The Weston to Kitchener trip takes a leisurely 1 hour 40 minutes.
I guess it would be handy for Oktoberfest!
As a result of the new extension, the Georgetown Line is being renamed the Kitchener Line.