There would be a force towards the outside of the curve, yes. But that doesn't mean the train would derail: in the extreme case the flanges on the wheels would

There would be a force towards the outside of the curve, yes. But that doesn't mean the train would derail: in the extreme case the flanges on the wheels would prevent it, but what actually happens in normal circumstances is the same thing that keeps trains centered on straights. In a nutshell: since the wheels have a larger diameter on the inside than on the outside, and since the left and right wheels are on the same solid axle and thus rotate at the same speed, any deviation caused negative feedback. The required superelevation for a given curve to eliminate lateral forces depends on the speed of the train; since not all trains travel at the same speed, and you have to account for slow or even stopped trains, the superelevation is always a compromise.