Skipshock

"Margo is a troubled schoolgirl. After the death of her father, she's on her way to a new boarding school in a new city. They never should have met. But when Margo suddenly appears one day on Moon's train, their fates become inextricably linked. If Margo wants to survive, she has to pass as a traveling salesman, too--except it's not that easy. Move north on the train line and time speeds up, a day passing in mere hours. Move south and time slows down--a day can last several weeks. Slow worlds are the richest ones: you live longer, your youth lasting decades. Fast worlds are sharp, cruel, and don't have time for pleasantries. Death is frequent. Salesmen die young of skipshock. That is, if they're not shot down by the Southern Guard first. As Margo moves between worlds and her attachment to Moon intensifies, she feels her youth start to slip between her fingers. But is Moon everything he seems? Is Margo?"--