The First Visitation is a significant event in the world of The Final Station.
Summary
What is known about the First Visitation has been culled from highly suspect official government reports, and the reports of those few who survived direct contact with the infected Zombies that resulted. What is known is that the event came out of nowhere - large Capsules appeared across the planet, spraying out a mysterious, unknown gas, which infected those who came into contact with it, causing a condition where inky black liquid leaked out of their orifices. Once infected and turned into Zombies, the victims quickly became ravenous, attacking anything that moved and was not infected like them.
In response, the governments of humanity bonded together under a large Council, and began to eradicated the infected best they could. Human settlements remained sparse after the apparent "victory" after the First Visitation, with small settlements being the norm rather than large cities (though there are some exceptions, typically centered around large military or scientific facilities).
Fearing a Second Visitation, the Council created the Guardian Program, a program to create a bipedal armored unit capable of destroying any capsules or invading forces. The governments of Earth initially argued as to who should have central control over the system, until it was finally decided that a central thinking unit would be created independent of any country's influence. This move angered many citizens who felt the bickering over the construction of the Guardian wasted valuable time - and still others who thought the program was a waste of funding and time in the first place.
While traditional travel remained untenable, with most roads destroyed and air travel too fundamentally expensive, rails became a predominant method of travel between cities. Within cities, where roads were in better condition, car and bus travel are still popular, but limited due to fuel shortages. Humanity depends on these tracks, and thus, most settlements are directly connected to a track.