Hi Ed, here's another screenshot to help explain what you would need to do for building a tube result.

After you build some surfaces with the right segmentation on the rail curve, you'll have pairs of pieces that will look like this:



Then you want to use the Edit > Trim command to slice off that inside area where they poke through each other.

The regular way that you would do this would be to select both surfaces, then run Edit > Trim, then at the cutting objects prompt you just right-click or push "Done" (or hit the Enter key) without picking anything else - that signals to trim that you want to do a "Mutual trim" where each object is both being cut and also acting as a cutter to other objects as well.

Then you would either pick the little pieces that you want to discard, or you can pick the bigger pieces that you want to keep if you set the Mode option to Mode = Keep.

It's still a bit tricky because for some reason the mutual trim in a couple of cases can have a somewhat messy intersection with a bit of a gap between the pieces. You want to be zoomed in a bit so that you can see the intersection fairly well when you are doing the trim and if you run into one that does not look good, then bail out of trim and select both surfaces and run Construct > Curve > Isect which seems to be able to generate a good intersection curve between them. Then use that curve as the cutting object in Trim to cut the surfaces.

Anyway, the thing is that when you've got 2 closed circular tubes that cross each other like this, there is a bug in the surface/surface intersector that tends to miss one of the 4 possible intersection curves. But when you are dealing with open surfaces like this instead of totally closed ones that avoids that bug and that makes it a lot more feasible to make it work.

- Michael