Parts of Montreal's underground city are notoriously confusing to navigate. That might have something to do with the fact that there's not much in the way of great maps of the underground.
The map I posted just above is the clearest one I've found. Just follow the orange line. If you can. See, even when armed with this map, one created for underground art collective Art Souterrain, it's all too easy to have dropout moments.
You have to keep in mind that the underground city is 3D, ebbing and flowing on different levels and floors in roundabout ways with vertical overlaps which can't possibly be shown on a 2D map. It personally took me about a decade to comfortably move from one end of it to the other and even today, I still have a couple of weak spots on the circuit that trip me up.
Why is Montreal's underground city so bewildering? It's a maze. That's why. Zoning issues and uncooperative parties forced developers to create unintuitive underground detours and oddball connections to please every land and building owner involved in order to avoid any legal snafus. Here's a bit more about the history.