Train track letters (Brio and Lillabo compatible)

This is a set of letters that double as Lillabo/Brio tracks, so that young ones might use them to form words.

The SCAD file does not include all letters (yet). I expect however to have some problems designing a few of them, such as F (hard to make it different from P) or C (will look like G).

Given the way this is coded, it is a simple modification of the code to turn any shape into a Brio track element!