The first O9 model I ever really built was the Alphagraphix card kit of a Romney Hythe & Dymchurch Railway 'teak' coach. Probably not the best starting point as a 20-seat coach is a long item for a lot of 15" gauge railways.
Now imagine a train of 20-seat coaches, with an O9 RH&DR Pacific at the head... that is the vision of Jonas Sommer, German engineering student and RH&DR volunteer. Using CAD and 3D printing has resulted in stunning results.
Jonas shares his techniques and results via his Facebook page "Kent's Mainline in Miniature" - which you can access without logging in if that isn't your thing...