What keeps a train on the track?

Here is an interesting video I found Watch here:

Here is the explanation I came across - "The inside lip is only a safety device, if it was only thing keeping the train on, wouldnt there be a loud noise all the time right. if you look closely (again) you will see that the actual wheel is sloped, meaning that the circle area on the outer rim is smaller then the circle area on the inside (beside the lip/flange), so whenever a train is going round a corner the momentum makes the train shift either left or right depending on what direction the corner is going. by this shift the wheel on the inside has a smaller distance to travel then the outside which is on a bigger circumference. thus bring the train round the corner without the use of the lips. the train sways to and fro and eventually staightens out again."