You aren't going to like this. Wider curves are the best curves. Body mounted couplers are the best mount. Purchasing top of the line train equipment in any scale is the best option. With all that said. I use, disclaimer and I don't expect you to. 15" radius curves as a minimum for my N scale yard tracks. 20" radius curves as the minimum for my main lines. I have one 24" radius curve on my mainline that trains run through and look amazing. Minimal overhang. I use #6 switches as my minimum. You won't find #4 switches on my main lines. Industrial switching...maybe. As far as what is the tightest radius curve you can use. Wrap a pieced of flex track around your little finger. What the heck!! I said gritting and clinching my teeth. I can only hope that helps. Take what you will from here and discard the rest. See signature.