I think I may have inadvertently given the reason (if by why the Northern had so many, you mean why so many railroads used that name for their 4-8-4's) when I mentioned the 4-6-2 was universally called a "Pacific" wheel arrangement, because it was developed in NZ (so the claim goes). They didn't so refer to it, as an NZ, because no doubt, most of the world wouldn't have known where the hell New Zealand was anyway at the time, unlike, say the 4-4-0 "American" developed much earlier (personally I think invented is too strong a word for something that came after the first successful steam locomotive), an iconic wheel arrangement. They couldn't call the 4-8-4 an American could they, so having been developed in the northern parts of the USA (Forget Canada, it doesn't count), Northern was as good a name as any. I did also say that NZed's 4-8-4 was referred to as a Northern, and I don't think you can get much further south than Invercargill, down at the bottom of the south island for railways, unless Argentina had Northerns that ran down to Tierra del Fuego........I'm wrong, you can get to the "End of the World" by train, to Ushuaia in fact, and by steam, check it out on Y-tube. Didn't see any Northerns though.