It's really hard to say, isn't it? If I remember rightly (and perhaps I don't...) although the different sections that ended up in Cabin Essence were recorded under section titles: 'Home On The Range', 'Who Ran The Iron Horse' and 'The Grand Coulee Dam'... their associated recording documentation always said that those sections belonged to a larger whole called Cabin Essence, which suggests that by the time of the recording at the very latest, Brian saw them as parts of a track that was distinct from H&V.
That doesn't, however, preclude the possibility that he composed some or all of those sections possibly with H&V in mind. That might explain why some of the track and BVs to the Cabin Essence chorus sound related to the later H&V Intro. Maybe the CE chorus (or something like it) was going to be part of H&V at one point before October 1966, at the writing stage.
It seems pretty clear that by the time the sections were actually cut, though, they were for a separate track.
So as with a lot of SMiLE stuff, if you're trying to be historically accurate, it depends when exactly your H&V mix is supposed to be 'from'. Is this a May 1966 mix? A notional 'September 1966' version, sounding like what you think Brian might have had in his head when he wrote some of the sections? An October 1966, December 1966, or January, February or March 1967 version? Because I think the one thing we can ALL agree on is that had versions of H&V been completed in any of those months (and who knows, perhaps some of them, or parts of some of them, were...!), they would likely all have sounded rather different from one another.
I know what the June 1967 version sounds like. But that's all I can say!
Of course, if you throw the 'trying to stick to history' aspect out of the window, you can put Who Ran The Iron Horse where you like! All the more so if you make the kind of mega-edit/mix where bits of H&V, Cabin Essence, Do You Like Worms, TOMP/YAMS etc etc all turn up throughout one gigantic through-mixed track. But that approach is not everyone's cup of Reddi-Whip...!
I'd say Fire, H&V Intro and the Cabin Essence chorus *are* all musically related by dint of the minor-scale arpeggios that appear in all of them (though in Fire, the notes aren't stepped, but played glissando on the strings)... but I'll admit that not everyone would agree with me on that, particularly as those parts are played by different instruments and in different keys on each track. You might say, 'well, those parts are just the kind of musical stuff that Brian was coming up with at the time'... and I wouldn't really be able to argue with that.