I can find all kinds of videos showing and possibly proving some locomotives will or can operate over HO 18 inch radius curves. Not something you will find me recommending. Not after some of the things I learned from my layout similar to the one above. I think I hear someone asking what is a radius? It the measurable point from the center of the circle, in a straight line to the outer edge of said circle. Clear as mud yet? Now with model railroading that radius is determined from the center of the circle to the middle of the track. What do I recommend? Keep in mind these are just guidelines. What experience has taught me: OOO The exception to the rule as they measure from one side of the circle to the other. 72" Minimum radius curve. S Measuring from center of the circle to the center of the track. 32" Mainline 28" Minimum radius curve. HO 30" Main Line Curves 24" minimum N 24" Main Line Curves and 15" minimum. Z 20" Main Line Curves and 11" minimum. Best curves on any layout, are the wide curves. You can use and experiment with the tighter radius curves. You will eventually learn why I chose the larger radius....long story... and enjoy your layout more. When your trains derail through track switches, jack knife and derail in reverse moves and stall or loose electrical contact, it's not fun. I have some pictures in my archives illustrating the differences of various curves in N scale. I had them posted in a album at one time and I think they disappeared about the time they shut something down here. Now I have to find them....again.