The barrier segregating Montreal West from the Ville Saint Pierre district of Lachine is here to stay. The Quebec Court of Appeal this week upheld a lower court ruling that Montreal West was within its rights to setup a barrier to car traffic between the two towns. Though Montreal (which the Lachine borough is part of now) may appeal, I'd wager their chances of getting heard at the Supreme Court level are slim. If the barrier comes down, it'll be because of a deal among neighbours, not because a hand was forced by the courts.
Montreal West argues this isn't about building a wall between rich and poor (there's no restriction on pedestrian travel), but the only issue is safety. I couldn't find any evidence of a problem when I checked it out two years ago. But it seems to be enough to convince people that it's necessary. And that's why it's the same argument used by other cities who erect barriers between neighbours.