Lionel 21" passenger cars will negotiate O48 curves, however that is about as tight as you can go. The kinematic couplers actually helps since they can stretch out a bit. I do prefer 18" cars, but made an exception for these since they don't make them in 18" and I don't want RK ones.
Below is a video of my BN E8's and 21" BN passenger cars on my O48 loop.