One idea I've heard of before was really neat.
A fellow I knew long ago was doing British VSF - only he tried to make it so that all his British units corresponded to real regiments in the British Army. Then any mechanical devices were painted (as you suggest) like locomotives (solid colours, with a binary or monochrome scheme, brass, copper, or gold trim, etc.), only in the appropriate regimental colours rather than anything from a particular railway.