I have been kicking around an idea for a while, my idea is not really post apocalyptic but it could be altered to suit.
an asteroid has hit the earth and the resulting dust cloud has blocked out the Sun creating a nuclear winter and plunging the earth into darkness, which means that vampires now rule the earth and have starting farming the humans left alive.
a small pocket of resisitance is fighting back against the vampires and has set up a underground railway to antarctica which is the only place on earth still uncovered by the dust cloud.