TMXU...Mexico and Canada specific containers???

I got a rail specific question and not really a trucking question I'm curious about. I thought Norfolk Southern just used it's regular EMHU containers when shipping to Mexico but apparently not. They are using new TMXU containers to do so. I've seen Schneider and several other carriers pulling these containers from Greencastle PA since it's part of the Crescent Corridor which these containers are part of. I'm kind of curious why Schneider doesn't use it's own containers for this and is instead contracting to use Norfolk Southern's containers especially considering they have a deal with CSX similiar to how JB Hunt has a deal with Norfolk Southern. I got to ask why they don't use CSX for this over Norfolk Southern as well. Also I got ask does Canada have something similar where they have a specific container for crossing the US Canada border as well. I thought those were just regular old EMHU containers. I drive for JB Hunt and we have our own containers and chassis and don't use anything else is why I'm curious about this all.