Hi Mike.
Now I know this is going to sound stupid but have you thought about banking the helex? Super elevation I mean, although you could bank the whole thing like a race track, like the old track at Monza or the super speedway ovals the "septics" race around, as its hidden it wouldn't mater if it looks as stupid as it sounds. Works for the real thing as it helps to mitigate the centrafugal force and gives higher speeds through the curve. So it should have the effect of lowering the resistance and thus increase the hauling capacity of the locos. You can exagerate the degree of banking to a point where its not quite tipping over. Just don't stop on the helex and it should work out fine although I would fit some sort of barrier on the inside so that if the worst happens your prize stock won't hit the deck. Just a thought haven't tried it myself you understand but as you've come this far its got to be worth a punt.
Regards Lez.Z.