If you're talking ocean freight out of Halifax, the answer is simple -- discharge at another port. Most of the Halifax callings hit NYC, so really it's an easy move to reach your Toronto/Midwest/West markets via another routing. Sure it'll have a different cost but likely speaking it's a temporary re-routing and most steamship lines will re-manifest the cargo due to the disruptions (similar to force majeure) rather easily.
The other most likely scenario for volumes in the already network is to crossdock from ISO & 53' domestic boxes into dry van trucks either at Halifax or further along the network. CN did move boxes further along (Charney and Moncton) to relieve capacity at Halifax, but I'm sure too some of those boxes have been deramped and crossdocked rather easily with the established trucking partners. Sorry if I suggested draying 1100 miles but that's not what I suggesting (re-read what I wrote, moving to the nearest ramp which would be Montreal to Toronto area)... you wouldn't do that if for anything the chassis migration, but I have done some 500-600 mile drays before with extreme circumstances. My suggestion was the crossdock into vans to fill the gap... Might need to pay to repo some trucks from other markets to boost capacity, but you can keep things moving and minimize delays on that solution. I've done this dance quite a few times, it's never fun but sometimes needs to be done.
Putting boxes into dwell or storage is definitely an option too if the product isn't time sensitive and the shipper can eat whatever impacts. That's a risk since we don't know how much longer the blockades will be, but updates from CN today indicated they are optimistic this will be over by week's end.
So to my first point on intermodal, which is margin sensitive anyways, it's really tough to say we're going to firstly, reduce from doublestack to single stack, then secondly we're going to alphabet route it (everytime you hit a rail interchange expect to add $500-$1000 to the car move) on what is already considered a short-haul intermodal lane (Halifax to Toronto) involving multiple shortlines and at least 2 Class Is. Surely stranger things have happened but I put this one into the unlikely category.
But to bring this back to CP/CMQ (I guess we are in that forum), I wouldn't be surprised if this pushes more shippers into an Atlantic Canada dual rail carrier strategy. Yes, CP is blocked on the Adirondack sub to CMQ right now (probably will be done soon too), but I'm sure once CP is in full reigns of CMQ this summer, they'll be able to point to multiple CN service woes this past year (blockades, strikes, ect) while touting the new Saint John to Toronto service, which seems to be primarily targeting domestic retail traffic. I'm sure IMCs will get involved soon since CN is 1.) their current only option and 2.) now a stronger competitor with their integration of H&R as well as TransX (which is kind of scary to be honest).
CP made clear their strategy last investor call not to purchase IMCs, but to greater integrate with them into the CP network as a more core network, so the last missing piece is Atlantic Canada. Also, look at the letters of support for the transaction, and 9 of those were from IMCs.