I mean let's be honest here, the plot kinda writes itself for this one too...
After every dragon cycle, the world of Tyria enters a brutal ice age. All life is dead except for those who boarded a train, built by an engineering Asura, that endlessly circles Tyria.
Those on board each own "train cars" homes which they can endlessly customize and decorate. Each month those with the best decorated cars move to the front of the train, while the person with the worst...their car is released from the train and lost into the frozen wastes.