The Railsea

The railsea is the main setting of the entire book, except the very ending.

History
Some time ago, there was a massive corporate war during which most of the seas were drained & rails layed down over the whole world, save for some "islands" in-between. This event is known as the Godsquabble, that created the railsea: the "sea" of rails covering most of the world.

Nobody really knows exactly how long ago the Godsquabble was, however there is a great amount of myth & legend surrounding it (& not everybody on the railsea believes it happened at all, & there are plenty of different versions of the story). It is, also, stated that it happened thousands of years before the start of the book. In that time, the people of this rail-covered world have built their cultures, traditions, mythology, & religions around the railsea.

Ecosystem
The ground between the rails is not only considered taboo--it is actually dangerous. Due to god-knows-what, many animals that are harmless in our current era are supermassive & murderous in the railsea future; the underground is inhabited by various creatures like burrowing owls big enough to wrestle trains off their tracks, ants & earwigs & worms bigger than people, ferrets & burrowing bees the size of cars, & moldywarpes; moles the size of big city apartment buildings. All of these creatures can & will kill people, if given the chance. Herbivorous creatures do exist, but as the narrator would put it, it seems as if they were placed under the railsea by the gods as a very mean-spirited joke, implying they are very much prey to almost every other burrowing creature.

Moletrains
One of the many types of train that roam the railsea is the moletrain; trains that hunt moldywarpes for many purposes (such as meat, leather, & bones). They play in this era a similar role to that of whaling ships in the late 18th to mid-19th century. The Medes, the most significant one, is the workplace of both protagonist Sham & tritagonist Captain Naphi--as well as many major supporting characters, such as Vurinam, Mbenday, & Dr. Fremlo.

Philosophy
An affectionate parody of Moby-Dick, the idea of 'philosophy' is a common one amongst moletrain captains; while in Moby-Dick, Ahab's obsession with tracking down & killing one particular animal was (aptly) considered strange & very disruptive, in Railsea, it is normal for moletrain captains. Having such a revenge-focused monomania is referred to as having a philosophy (such as Mocker-Jack being Captain Naphi's philosophy). Most people who are not captains find it very difficult to take their obsessions seriously, save for some, such as Captain Naphi, who are widely reknowned for having particularly interesting stories with theirs--however, readers have no idea what made Mocker-Jack her philosophy, as with most captains, it's a missing limb, but Naphi's seemingly-missing arm was revealed to be fake near the ending & her true motives behind hunting Mocker-Jack were never revealed.

Basically, every moletrain captain is a Captain Ahab wannabe.

Salvage
Salvage is the railsea-era equivalent of archeology; salvors, such as Sirocco & the Shroake siblings, search & dig all across the railsea to find items from hundreds, thousands of years ago (called arche-salvage), recent enough eras to be worth less than the other forms (called nu-salvage), & the most valuable type; trash from alien worlds, that often refuses to obey the laws of physics (called alt-salvage). Salvors often sell these things for varying amounts of money, with alt-salvage being the most expensive, & nu-salvage being anywhere from cheap to practically worthless & only useful for collectors. In Manihiki, there is a rather famous salvage-market called Scabbling Street Market, though most other salvage-markets are temporarily held by the crews of salvage-trains if they are feeling particularly generous &/or have a lot of time on their hands while docked.

Pirates
The pirate trains on the railsea are similar, of course, to pirate ships. They are mentioned frequently, though the only one seen is the Tarralesh, which captures Sham for information on the Shroakes. The Tarralesh is blown up by the Manihiki Ferronavy.

Manihiki Ferronavy
These military trains from Manihiki are known to be rather belligerent, tending to pick fights with other trains for no apparent reason. They have weapons & are powerful enough that people don't tend to mess with them, but nobody really likes them, as they throw their weight around far too much. They are responsible for blowing up the pirate train Tarralesh, though they are also responsible for destroying the Bajjer's crops.

Heaven & That Apt Ohm
Somewhere in the deepest darkest parts of the railsea that nobody even knew existed, there is a heavily guarded town: Heaven, the place where the Godsquabble happened as well as the place where the now-degenerate descendents of the original god-squabblers live, such as That Apt Ohm & his subordinates. Since the railsea was first laid down, its denizens have owed an ever-increasing amount of money to the citizens of Heaven, however, eons later, such a sum of money simply does not exist, & Heaven & its people have been reduced to myth. On the side of the world where Heaven is, it seems as though part of the original ocean remains un-drained--it is a mystery how nobody knew this. At the ending scene, Sham tells That Apt Ohm that the bill will never be paid, & they fight, & That Apt Ohm is killed.

Trivia

 * "That Apt Ohm" is an anagram of "Topham Hatt," a reference to Thomas the Tank Engine.
 * "Moldywarpe" is not a made-up word. It is, in fact, a medieval word for moles.
 * One of the gods is called "Railhater Beeching," a historical reference to Dr. Richard Beeching, who was notorious in Britain for closing a lot of railway lines.