Mortal Engines by Philip Reeve

London is hunting again. Emerging from its hiding place in the hills, the great Traction City is chasing a terrified little town across the wastelands. Soon London will feed. 

In the attack, Tom Natsworthy is flung from the speeding city with a murderous scar-faced girl. They must run for their lives through the wreckage -- and face a terrifying new weapon that threatens the future of the world.