My son always builds lego bridges for Brio train tracks, is annoying to see these fall after all the work he has put on building them. The block underneath still locks in other duplos. Note this work can not be used for commercial purposes...The brio track profile is build with points and paths, nice to make it parametric (I have another piece work in progress to make this profile extrude along a path...). There is a "u" module you can try too, but is not necessary. The female coupling could be a little more tight, but up to you to modify openscad parameters.