The minimum curve radius for any HO scale layout must take into consideration the type of rollingstock that will be running on it. For up to 50 scale foot carriages, and 4 axle deisels, and up to 0-8-0 steam locos, a minimum radius of 18 inches is best. For larger steam locos, 6 or more axle deisels, and 80 scale foot passenger cars, you would probably find that you need a larger minimum radius. I have adhered to the 18 inch minimum radius standard for my layouts, but some people use even a 12 or 15 inch minimum radius. The sharper the radius of the curve the less rollingtsock will run on!