In the city of Montreal itself, what areas are good for railfanning? Outside of the city, I'm probably going to go to Dorval, St. Catherines, Deux-Montagnes.
Montréal-Ouest station is also a good spot for railfanning. Three lines converge at this station (Vaudreuil-Hudson, Blainville-Saint-Jérôme and Candiac). Easily accessible by train and not very far from downtown.
Saint-Lambert station (near Victoria bridge) in Longueuil on the south shore is also a good spot for CN, VIA, Amtrak and AMT trains.
Near downtown, there is the Wellingtion street bridge, over the Lachine canal, where you can see a lot of VIA and AMT trains.
As far as AMT train riding goes, I'm going to ride to Deux-Montagnes, and either one of the diesel lines that VIA rail doesn't run on or along. Maybe go to Cantic, or up to St. Jerome, and then a bus to the Metro.
I think you mean Candiac. There is no AMT line named "Cantic". I would suggest the Blainville-Saint-Jérôme line, because it's easier to do a round trip by train. Also, there is a frequent express bus service between Montmorency metro station and Saint-Jérôme train station, via Blainville and Sainte-Thérèse stations (CIT Laurentides route 9). I don't think there is a similar bus service along the Candiac line.