Many moons ago I began planning and tracing the spaces between the roundhouse bay tracks. Research plausibly indicates that roundhouses did have wooden flooring. I chose this Northeastern 1/8" wide scribed siding. With a few more sections to cut and install, I am also adding a loco pit where workers clean loco undersides. Clearly more to do yet filling in end gaps like the one below. Next big project here is how to add lighting, most likely on beams above the bays. 
