I have used very light shapeways four wheel mine carts in Nn3 (Z) as runners (not much bigger than a 4 wheel truck) and found that in spite of being ridiculously light the use of metal wheelsets which places a little extra weight in just the right place had them tracking 100% reliable including through turnouts. That was in a 10 car string which is all I tried. I suspect it would go at least double that. The biggest difficulty was placing and coupling them but I just held the first one down with a toothpick while placing and coupling the second one.
Tillig makes HO/HOn30 mixed gauge track. However they don't have a complete 3-rail turnout. Word of advice, look outside the US for equipment. There is a lot of stuff in Europe and Japan which could find a ready use in HOn30. Like this railcar set www.1999.co.jp/eng/10408883 (out of stock but if you look in the web or in Yahoo Auctions Japan - through a broker, there are several, otherwise you'll get stuck with the language - you might be able to find one) - although of Japanese prototype it could find a ready use in an US. Shapeways and iMaterialise are also good sources, there a loads of people offering 3D printed models. Cheers NB
I second Nick's comments. If your not married to the idea that it has to look like Colorado or Maine, there's alot of good looking HOe equipment out there. Tillig, Liliput and Roco makes some good stuff.