Just another McDonald's franchise restaurant, but located very conveniently in Union Station (GO and Via Rail train terminal, not in any fare paid zone). It's good for grabbing a bite before commuting on GO, taking a longer trip on Via, or just walking out to downtown or Metro Toronto Convention Centre, etc.
As you can imagine for a train station, the volume of customers is high. They do managed to handle this torrent of people and the response time is average (not terrible but not immediate).
Getting seating indoors is not easy; there are some benches near waiting areas but none specifically for restaurants. But you're likely to run off somewhere anyway, since this is Union Station after all.