Emailed to Nicholas, is this helix what you're looking for? The outside track is 10' diam. and the inside track (as sidings) is 9' diameter. See my Four Level Track with Helix Access for how to information.
My MTH Big Boy pulls long freights (60 car) up the helix with ease. If your building one that large I have the cut out drawings for the base parts. I'm happy to share the drawings.