Hello Michael,
thank you for your explanation, now it is clear.
Also thank you for polyhedron tips, I will try to use them to keep the model easier to manipulate. My experience is, using a lot of NURBS surfaces slows down the rendering and manipulating. I am doing a lot of Boolean operations and some of them will fail with non-planar surfaces, or result in losing the "solidness" and change to joined surfaces.
I also played with parameters at .stl export, and found better to use more polygons and let the printing software to "antialias" the surfaces.

Michael, you are doing a good job, I like MOI and thanks MOI I can build nice railroad models :-)