Answer:
One is safe and civilized, and the other is savage.
Step-by-step explanation:
This is the main difference between the setting of the Southern House and the setting of the train. The Southern House where Buck lives is comfortable and safe. This represents civilization, and all the things that this brings to dogs like Buck. On the other hand, as Buck gets away from the house, he also begins to get away from civilization and its comforts. The setting of the train contrasts with the house as it is a savage and uncivilized environment.