The top of the wheel is moving faster than the train. If the contact point between the wheel and rail is not slipping, the relative velocity between the wheel at that point and the rail is 0.

To maintain that condition, the top of the wheel has to travel at twice the speed of the train. Imagine a point on the wheel. Each time the train moves ones circumference forward, that point also makes a circular trip of 1 circumference.

So at the top, when the motion of the point relative to the train is full forward, the net velocity is twice the train. At the bottom, when the motion of the point relative to the train is full backward, the net velocity is 0.

There are diagrams and further explanation here: http://physics.stackexchange.com/questions/48234/why-is-the-...