The first law of motion, as you learned in what keeps a bicycle upright, is that a moving thing goes forward in a straight line. If the direction of the moving is changed some other force must come into play. What force will interfere with a forward movement on a level? Why a rise in the level. It is harder to go up hill. To make a little hill the outside rail is raised quite a little higher than the inside rail. The height to which the outside rail needs to be raised depends upon the sharpness of the curve, and the speed at which trains are expected to run around it. Around very sharp curves trains must "slow down" to keep from running off the track. When you run around a corner, or ride around one on a bicycle, you lean toward the inside of the curve. Horses do this, too. Just take notice and see if this isn't true.