I have to plead ignorance on the Exact Canadian regs, but I know from experience that CMQ participates in interswitching both to and from SLQ at Lennoxville and CP at St-Jean.Shortlines aren't applicable to C-49 (Interswitching) rules and also if they were this area lies within the Quebec-Windsor corridor. SLR is mostly a paper railroad for CN though, but still within the exemption zone.
Domtar and Kruger are open on SLQ and the exEKA plant is open on CMQ. MMA complained vociferously to CTA about the negative effects being forced to interswitch for CP and CN had on the company.
On the CP side CMQ can hit St-Jean, Delson and Seaway Spur while Iberville, St-Jean and Versailles are open to CP on CMQ. CP didn’t interswitch CDAC or MMA due to a traffic agreement, but that changed after the CMQ creation.
MMA complained vociferously to CTA about the negative effects being forced to interswitch for CP and CN had on the company. You could find those filings online at one point but I can’t find them at the moment. If you are still friendly with someone in CMQ marketing, don’t take my word for it confirm it with them.