The line formed by 55th Ave., Côte de Liesse (Highway 520), Highway 13, Côte Vertu Blvd., Sauvé St., and Pie-IX Blvd. represents what?
UPDATE: A bunch of you got this right, though NDGer was the first. It's the border between the AMT's Zones 1 and 2 (grey and pink on this map).
The AMT's zoning system (where higher-numbered zones pay more to use commuter trains and multiple-network bus trips) is roughly based on the distance to downtown Montreal (assuming that everyone is travelling downtown to work, which of course isn't always the case).
There are some quirks though, for example Lachine is considered Zone 1 while Longueuil, which is much closer to downtown, is Zone 3. Similarly, Kahnawake (Zone 5) is closer than Pointe-Claire (Zone 2), and most of the Zone 8 territories are closer than Rigaud (Zone 6), though that won't matter once Rigaud loses its train service on July 1.
Thanks to SMS for this week's question.