I used FreeCAD to design a small part that would fit on top of my son existing wooden train cargo container and which allows to carry a car instead.
The FreeCAD file and STL part + other pictures are available at https://www.thingiverse.com/thing:2718451
I am extremely pleased with the result, it fits great and the whole process was very smooth. I just had issues when doing the side cutouts of having difficulties to locate which segment is which in the sketcher, as many overlapped (maybe there is a better method to do it).
It is one thing to have a 3D printer, it's another to design your own parts. Seeing your parts come to life is really an incredible feeling!
Thank you very much guys for putting together the great product that is FreeCAD and for supporting it so well!