Just spent the last couple hours cleaning the room of junk and taking measurements. It looks like I can get a 30 x 30 helix down by the closet door. This will allow the door to open and also keep the helix away from the window. So if the math is correct; that will let me use 13.75" unitrack to make the single track helix (which I have on hand).
There was some talk about train length. No more than 15 cars with a optional caboose I'm thinking. Some shifters may only be 5 or so cars. Depends on the tipple be served. Why a caboose? I'm thinking of a tipple that requires a back up move and NS/CSX still use cabins for such use. Yes back up moves with truck mounted couplers is certain doom but I think if the speeds are nice n slow and the move is short, I "should" be ok. Famous last words.....