Montreal (YUL) is a single terminal with three 'jetties' - Domestic, International and Trans-Border (US flights). There can be a bit of walking depending on your connections but nothing too difficult and you certainly don't have to take shuttles or trains.
As for factoring in a Marxh snowstorm, good luck with that. If it's bad enough, there is a good chance you will not have to worry about a connection in YUL as you may not actually get there. On the plus side, if you can land in YUL, there is an excellent chance you'll be able to depart as they are very good/experienced at managing snow.