The sensor was small enough for an N-gauge train, which translates to 1:148 scale or about 9mm from rail to rail. His idea was to build a tiny control board that could fit inside the locomotive ...

And the simplicity of using N scale locomotive mechanisms to model HO scale narrow gauge equipment has made HOn30 (also called HOn2½) a well-known modeling scale. If you want to model in HOn3 ...