727: Mainline

For two reasons, most experienced programmers rarely use internal pointers to create lists:

• When you use internal pointers to create lists, you must add a new member variable to the class definition that defines the objects you want to tie together. You may not have access to that class definition; moreover, you always should be slow to muck around with existing, debugged class definitions.
• You may want to include particular railroad-car class objects in more than one list, such as a train list, a box-car list, a full-car list, or an old-car list. It would be incredibly awkward to add internal pointers for each such list.