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To get started, you decide to revive part of the railroad-car class hierarchy that you learned about in Chapter 15:
*----------------*
| railroad_car |
*----------------*
^ ^ ^ ^
| | | |
*----------------------------------------* | | |
| *---------------------* | |
| | *----------* |
| | | |
*----------* *----------* *----------* *----------*
| box_car | | tank_car | | engine | | caboose |
*----------* *----------* *----------* *----------*