The most organised band of survivors is the Amtrak Federation. Four hundred American citizens survived the nuclear war in a vast underground bunker beneath Houston, Texas. Over time, their descendants established additional underground bunker-cities (known as divisional bases) linked by a high-speed subterranean rail network. Ruled by a self-perpetuating dynasty known as the First Family, the Federation expanded slowly in numbers and territory. By AD 2989, when the novels begin, the Federation sprawls across much of the south of the United States, from Colorado and Arizona to Mississippi. It is technologically advanced, employing microlight aircraft and 'wagon-trains', massive, 600-foot-long vehicles that act as land-based aircraft carriers and mobile HQs for heavily armed infantry personnel. The Federation's personnel are heavily susceptible to above-ground radiation and rarely live past their fortieth birthdays as a result.