I've always desired a small engine to run on my small door layout. Lots of looking here and elsewhere online provided plenty of inspiration. My three main goals were a) make it powered by PF b) make it similar to something that exists IRL and c) make it tiny! I've seen some small switchers but mine might be the shortest at just 14 studs long and 6 studs wide. I even managed to squeeze in a minifig (well, 1/2 a minifig). The gray plate on top serves as an on/off switch, though a curved side piece needs to be removed to see the light. Designing it was a bit of a challenge, and ultimately I took two potentially controversial shortcuts. This one took only about 5-6 hours to design in LDD. It contains about 175 pieces and cost about $80 US to build with pieces from BL including the PF parts.  Below I've included lots of pics and a link to a quick video of it running. It looks right at home on my layout and does a good job moving cars around, especially considering it's very light weight EDIT: Here's a link to my LDD file so you can make, modify, copy or inspect my design.  https://bricksafe.com/pages/sed6/switcher-shunter-engine Hope you like!