(Yes, I said "tootin'". On purpose. Cue groans.)
The setting, if you've missed every other review and comment, is the railsea: a vast trackless waste of flat ground, only it's not trackless because it's covered with railroad tracks. They snarl, they knot, they go nowhere and everywhere in every which direction. Nobody knows why. But to go anywhere in this world (between the "islands" and "continents" that jut up above the railsea) you need a train.
No, you can't walk. The carnivorous moles and giant antlions and burrowing blood-rabbits will getcha.
Upon this outre concept rides the mole-hunting train Medes, upon which rides the young apprentice Sham ap Soorap, among a crew of hardened molers with satisfyingly gonzo names like "Danjamin Benightly" and "Boyza Go Mbenday". Soon they will discover a wrecked locomotive which carries a secret that will send them to... adventures. Really, what did you expect? And there will be a giant white mole -- well, ivory-colored -- okay, yellow, if you insist.
It's not played the way you expect, though. Mieville (and I have to wonder whether this book was written just because people kept misspelling his name) would rather move his story along than hit you over the head with Moby Dick parallels. Oh, they're there -- he just doesn't feel the need to take them all that seriously. And the end of the world, that's not taken all that seriously either. I mean, it's there, you can't ignore it. But.
Railsea reminds me of George Alec Effinger, and not just the scene where Effinger rolled the Great Plains flat and Teflon-coated them into the world's largest Modern Kitchen Range of the Future. Mieville attains that gentle White-Knightish balance of whimsy and sorrow. Or finality and hope and resolution. Alternate future and alternate past. Oh, just go read it.