Märklin Plug short circuit protection and plug

The plugs known als “Märklin plugs” can get a short circuit when the screw get together. 

Here are two versions to solve this problem:

Version1:  A protection for already connected coupling and plug

Version2:  A protection that makes the couple and the plug separable 

Added an additional print profile with a 3‑slot version, as well as an alternative version with an extra locking pin.

easy to click in.