Paul says:

Your best bet is 22” or higher. You can use 18” on some brands of engines but just asking for a derailment.

Ken Holbrook says:

I have some tight radii on my layout that’s probably below 18”. I have 3 SD40’s in my collection. I discovered that one derailed every time they hit a tight curve. Another one derailed around 20% of the time, and the last derailed occasionally but hardly at all. To rectify this, I a permanent marker and marked where the trucks were coming into contact with the metal frame. Then I removed the loco from the frame and used my Dremel to grind out enough space to increase the truck “travel”. Now they navigate the tight turn with never an issue. They’d probably go down to a 15” radii without derailing. It’s import to remove the frame so you dint get metal shavings picked up by the locomotive. Some 6 axle locos are just not built with enough truck swing from side to side. The manufacturers could easily do this during the manufacturing process. Why they don’t is beyond me. Good luck!