Railsea (2012) by China Mieville. "On board the moletrain Medes, Sham Yes ap Soorap watches in awe as he witnesses his first moldywarpe hunt." So reads one of the publishers' blurbs for this Young Adult science-fantasy novel. Never let it be said that Mieville would dumb things down for a younger audience!
A 'moldywarpe' is a term for a whale-sized mole, by the way.
Set on a future Earth separated from ours by an unknown but Sublimely vast Time Abyss, Railsea follows the young Sham's voyage from doubtful surgeon's apprentice to a mostly confident explorer over the course of what seems to be about a year.
All his adventures occur on and around the Railsea, the vast and interconnected web of rail lines upon which the engines of many countries and organizations 'sail.' Mysterious machines dubbed 'Angels' maintain the Railsea. Gigantic versions of many of our smaller land fauna -- moles, ants, carnivorous rabbits, spiders -- hunt humans and are hunted in turn.
The Railsea is a great and intricate thought-experiment that seems perfectly believable. Mieville has invested this world with his terrific imagination, making it a startling yet weirdly familiar place. There's a quest, but it's for knowledge. There is our nominal hero, but he's joined by many others, all of whom serve a purpose (and all of whom serve Mieville's socialistic bent as a collective hero when taken in total).
It's really a marvelous book in terms of imagination, characterization, and style. One of the narrative conceits is that this tale is being told at some later date by a narrator who occasionally stops, starts again, explains some arcane bit of Railsea lore or language, or even apologizes for leaving one or another strand of the narrative for a lengthy stretch of pages until something important happens.
There's a long tradition of such novels as Railsea, set on a future Earth so distantly future that it hardly seems like Earth, from William Hope Hodgson's early 20th-century quest The Night Land through the works of writers that include Clark Ashton Smith, Jack Vance, and Gene Wolfe. Railsea is a worthy addition to their ranks, for young adults or anyone else. Highly recommended.