Ok,...
(wow, I go away for what was supposed to be 2+1/2 weeks, then turns into about 6 weeks and look what happens!)
I see a lot of good points in this discussion.
As far as spring vs non-sprung switches and or crossovers goes, there's an easy solution....
go with what we know WORKS now! The crews are used to the backup moves from AC already, so why change that? Why change the original config of the siding? I admit, the idea of a runaround SOUNDS neat, but would the added work and the questionable safety be worth it? Granted, it's not for me to say, but my vote would be "When in doubt, refer to the original manufacturer's specs!"
Maybe that's just the Navy in me, I dunno.
As for the tank and the fire train, I thik someone (Paul Horkey) is onto something here, but here's how I'd do (or try to do) it:
1) Fix the "spare" tank and maybe line it with plastic or add metal (weld/rivet?) to it or both, but "make it fast." (and by "fast" I mean the Naval term for "secure", not the common term for "expediant."

)
2) Once the siding is in place at TOM, place the tank convenient to the siding.
3) Dig a well for a pump, ala the Sheepscot WT, and use the "spare" tank to store water for filling the B&H tank, and possibly locomotives if.when needed. (I'm anticipating this could be a right-handy arangement once steam locomotives start working the Mountain grade.)
4) Station the fire train at TOM durring the times of highest fire-danger (as was previously suggested.)
5)Drain the tank at the end of each summer, prior to the freeze.
Well, that's my $0.02 worth. Mere opinion, far from "Law."
