Railsea

Savage giant moles, rail pirates, and explorers abound in China Miéville's thrilling young adult novel, Railsea.
On board the moletrain Medes, Sham Yes ap Soorap watches in awe as he witnesses his first moldywarpe hunt. The giant mole bursting from the earth, the harpoonists targeting their prey, the battle resulting in one's death and the other's glory - are extraordinary. But no matter how spectacular it is, travelling the endless rails of the railsea, Sham senses that there's more to life. Even if his captain can think only of her obsessive hunt for one savage mole.
When they find a wrecked train, it's a welcome distraction. But the impossible salvage Sham finds there leads to trouble. Soon he's hunted on all sides: by pirates, trainsfolk, monsters and salvage-scrabblers. And it might not be just Sham's life that's about to change. It could be the whole of the railsea.

China Mieville