Conceptual OOP Question - AArgh!

Thanks Bill, I get what you are doing with the train using the display methods of the carriages. This makes sense to me! Guys, when you plan your code, what guidelines do you have in terms of deciding what goes where? Aside from the one that Dreamora mentioned "...everything that modifies and affects this type should be created with functions in this type and not through something from outside" is there any other rules that you guys stick to and a newbie could use as a form of checklist?

Thanks again folks!