Plastic collar to wrap around front/rear magnets on Brio style toy trains. Design is based on the style used by Thomas and Brio brand trains, with the base of the cup between the magnet and the train. If you really want to produce an over-magnet version similar to what the BigJigs brand uses, just call magnet_collar(false). However, by placing a layer of plastic between the magnet and nail head, you considerably diminish the magnet's ability to hold your train cars together. Measurements are based on parts from Cherry Tree Toys. Note that they changed suppliers and the nails you receive will not be barbed like the one in the photo. Find the latest versions of this and other related designs at https://github.com/ex-nerd/3d Print and enjoy. I'll eventually get enough parts printed to do a writeup about how to put together a whole wooden train.