Final Fantasy XIII is set in a futuristic world controlling advanced technology and magical crystals. Pulse, the world Final Fantasy XIII is based on has had an evil history which has resulted in the construction of the floating utopian city called Cocoon. For a reason so far unkown, the citizens of Cocoon fear the world below.

The Crystals have the ability to create life forms and machines in Pulse. They also communicate with the super-natrual beings known as the fal'Cie who were responsible for the construction of Cocoon 13 centuries ago. The fal'Cie's descision to create this paradise was made to protect the people from an "infection" on Pulse.

In present day, the Holy Government in charge of Cocoon is extremely paranoid of people coming into any contact with Pulse. Anyone who is suspected of being influenced by the infection is subject to quarantine and is banished from the city. The prisoners on the train are most likely in the process of being exiled. Because of this, we believe Oerba Dia Vanille who is travelling on the train is suspected of being infected.