KD's work fine on my 26-28" radius (you can do the math...) tightest loop running a Weaver RS-3 and 40' cars as well as my NW-2 and all of my steam up through the 2-8-0's in 2- and 3-rail operation. I have one curve that is much tighter that limits use to just an 0-4-0 switcher. It's really dependent on your choices of rolling stock and motive power as well as era modeled. If you're going to ruin anything larger, you'll probably need larger curves or you're going to have "problems" using car mounted couplers that are limited in their swing motion by the their box. I put that last limitation in since the use of radius couplers in trolley operations permits the use of far tighter curves than one might normally consider possible.