I've seen it!
Yes, it's true that the Montreal Metro exists, but I don't know if you can call them "underground hallways" because what's down there is immense: Huge shopping centers. Three floor fountains. Restaurants. Bars and it's quite possible that people haven't heard of it because it's only in the center of the city and due to the poor underground network that the city connects the central stations of the same. What surprised me is that I can go up an escalator and suddenly appear in a crowded store :)