EMP boxes are brokered with shippers directly or via their 3PLs, they can be reserved via a platform called Rez-1. They started cropping up on the Moosehead last year in small numbers, but more and more have been showing up as of late. A lot of it is consumer goods going to retail stores and distribution centers in New Brunswick and Nova Scotia. EMP is simply a box pool owned by some RRs, similar to an container TTX they are free runners within the network of the box owner RRs.
Rail contracts for steamship lines are lane specific and usually come with KPIs. A steamship line can seek additional capacity on other lanes within their contracted rail partners, either spot or in short-term 'ad-hoc' agreements. So for example, CP has a long term deal with Hapag, but they move some volumes via Halifax on CN. Since CP doesn't hit Halifax, Hapag has either grabbed spot capacity or created an ad-hoc agreement.
The MSC Canada Gulf service that calls Saint John also hits Montreal, and the rail routings for that from inland have always loaded/unloaded via MTL and not SJ. Only rail moves so far from MSC at SJ came when labor dispute hit Montreal. This move of the Canada Express 2 is a diversion from Halifax, likely due to capacity/congestion issues either at the port or rail (or both). I would guess, and its just a guess, that it's a spot move that will probably go CP due to capacity issues with CN or again the port. It could definitely be an evaluation too for a future service, but I wouldn't be surprised if you see those boxes rail out via CP (and they will rail out one way or another) just because there is excess capacity and I'm sure they priced it as such.