Railhead Series by Philip Reeve

Come with me, Zen Starling, she had said. The girl in the red coat. But how did she know his name? The Great Network is a place of drones and androids, maintenance spiders and Station Angels. The place of the thousand gates, where sentient trains criss-cross the galaxy in a heartbeat. Zen Starling is a petty thief, a street urchin from Thunder City... moreCome with me, Zen Starling, she had said. The girl in the red coat. But how did she know his name? The Great Network is a place of drones and androids, maintenance spiders and Station Angels. The place of the thousand gates, where sentient trains criss-cross the galaxy in a heartbeat. Zen Starling is a petty thief, a street urchin from Thunder City. So when mysterious stranger Raven sends Zen and his new friend Nova on a mission to infiltrate the Emperor's train, he jumps at the chance to traverse the Great Network, to cross the galaxy in a heartbeat, to meet interesting people - and to steal their stuff. But the Great Network is a dangerous place, and Zen has no idea where his journey will take him. less

Zen and Nova have ridden a train from one world to another before. They come from the Network Empire, whose stations are scattered across half the galaxy. But this time it's different. They've passed through a gate which shouldn't exist at all. They did terrible things to open it, they don't know where it leads, and now they can never go back.Chandni Hansa has just been defrosted from the prison freezers by the new Empress of the Great Network... moreZen and Nova have ridden a train from one world to another before. They come from the Network Empire, whose stations are scattered across half the galaxy. But this time it's different. They've passed through a gate which shouldn't exist at all. They did terrible things to open it, they don't know where it leads, and now they can never go back.Chandni Hansa has just been defrosted from the prison freezers by the new Empress of the Great Network. When railwar erupts around them, the two find themselves running for their lives. But where can they run, when nowhere is safe?And all the while, the mysterious Black Light Zone is calling them. But as their lines separate and intersect, among trainsong and engine-roar, battle cries and gunfire, will they find a way home? If home even exists any more... less