Does the SW1 have a centering spring on the coupler connecting to the train? If so, the centering force might be a little strong and is dragging the lightweight car's wheels over the railhead just as it enters the curve. Another possibility is that the coupler on the SW1 just resists swinging a little too much regardless of any centering tension, so again, it's dragging the lightweight car off-center and causing it to derail.
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