16mm Narrow Gauge Ffestiniog Bug Box by Andrewlongman licensed under CC-BY 4.0
The files include sides to complete either first class, third class or observation cars. These are also available as DXF files if you have a laser printer.
I've not actually finished building any yet, but working on it: https://gardenrails.org/forum/viewtopic ... 54&t=13226