The author states that relativity is not new, and she uses a train as an example. Explain this example. Why does the author use this example?

When sitting inside a moving train, a passenger observes other passengers as stationary, while the scenery outside moves by at great speed. To an observer outside the train, however, the scenery is stationary while the train, and thus the passengers on the train, go by very quickly. The author uses this to illustrate that size and space are relative, while light is the only constant.