Inspired by other narrow gauge enthusiasts, I have built a compact motorised assembly using Technic with Circuit Cube's motor & bluetooth battery combo for a small locomotive.
In this first iteration, whilst standing on the shoulders of others, I have constructed a compact 4 wheel drive rail bogie. This is based on "Lil' Red" (based on the instructions from a talented designer on rebrickable, and his integration of Circuit Cube - links below) but with a different gear arrangement achieves higher speed.

https://rebrickable.com/mocs/MOC-6717...

   / mrasperka