What is going on?

I struggled to understand Einstein at all. It was kind of over my head. But one thing that I had a particularly hard time with was the train example on page 10. Is the point of this example to show that you can describe motion in two different ways, depending on how you view the motion? And if that is the point, does Einstein conclude that motion is relative to how an observer perceives it (ie. that there is no absolute motion)? How does the train example defy the principle of relativity?