It shows diagrams of various track plans, from simple loops to large, complex systems. You can choose the layout you like, then order the book with that specific track plan from Atlas. The plan lists everything you need, shows exactly how to position it, and includes complete wiring information and suggestions for scenery, etc. Lionel, the oldest O gauge maker, also has a large selection of layouts. You can check their website.
Most people buy trains to run.
A few buy as an investment or collectible. Others that are just as old are virtually worthless. It's all in the whims of the collector market. Even more amazing, some items gain in value only a short time after their production run is over. Others remain the same.