Dystopia and post apocalyptic tend to overlap, but there is a slight difference.
Snowpiercer falls more within the dystopia theme though since it focused more on the society and how dysfunctional it was.
The Road, Book of Eli, and Mad Max are all straight up post apocalyptic. If we're talking about the original Mad Max movie, that one is more "pre-apocalyptic" since it's showing more of the break down of society before the bombs fell.