Hi Ed, one area that the intersector can have trouble with currently is when 2 tubular surfaces cross over each other forming a kind of "crown point" intersection between them.

That's what you have in this case when you get both of those pieces interacting with one another:



Also it looks like there are some edges there (probably from a sweep curve that had the rail made up of multiple segments, which generates one surface per segment) which will likely add to the complexity of the intersection calculation in that sensitive area as well.

If you select those 2 tubes and run Construct / Curve / Isect, to generate their intersection curves, you can see that they are quite a mess:



So anyway, that's the reason why the boolean does not work right, the surface/surface intersector is not able to generate the proper closed intersection curves there.

Generally the easiest way to work around this problem is to have one tube be slightly larger or smaller in size than the other one, that tends to avoid the problem in the intersector.

I'll see if I can give you any other tips.

- Michael