When AC ordered their 744s in the early 90s, Transport Canada imposed a restriction: in the event of a massive troup movement operation, AC would be required to use their 744s to supplement DND's transports. Beacause of this, extra 'beefing up' of the landing gear and associated structures was required. Obviously, this added extra weight; in fact, this added weight severley restricted certain loads on longer flights. When AC parked their 744s in the desert upon delivery and desperately tried to sell them, there found there were no interested parties because on many flights, the extra weight made in uneconomical. AC reluctantly brought them into service, but have always had that weight restriction.
It sounds farfetched, but I assure you, I've heard this from many reliable sources. I may have some facts wrong, mind you.
I am unsure if ex CP machines have this mandated extra feature...