So in many cases, for each human that dies, a new member of the undead horde rises to join ranks. One becomes two, two become four, four become eight, etc. Those in the vicinity of the “zombie trains” suddenly dropped everything and ran for their lives. The result was a mass exodus of people without supplies to survive. The subsequent refugee camps turned into a free-for-all in just a few days.
Humans began killing each other over food, water, and other necessities. With the fall of the governments, no law enforcement authorities existed. Tribal gang warfare became the rule and only the strong were to survive.
Inner city thugs and gangsters rose to the top of the newly forming islands of society. Civilization would not be an accurate term to describe the new world.