If it's the legacy S-2, they were made with body mounted couplers. I have one in CNW. Look onthe box label for the minimum curve requirement. Might not be enough swing for such a tight radius. Adding weight to the freight car won't help as the engine is literally pushing the car towards the outer rail.
Look at the centering spring, sometimes the coupler doesn't want to move cleanly in one or both directions. If there's any force in moving it to the side, it'll pull a light car off the rails. The cure is either free up the coupler or use a heavier car for right behind the engine.