This was created to attach the train and the waggons for the Loco-motion set (https://www.myminifactory.com/object/3d-print-the-loco-motion-172). The attachment was missing in that project (or I missed something) so I decided to dip my toes and design my first item to 3d-print.
The attachment fits nicely into the train or the wagons and I did not need to glue anything.
For those interessted in how I did t I used www.tinkercad.com which is an online tool. I measured the space for the attachmetn in the train and used a square, a circle and a cylinder (as a hole) to create this piece.