A uniquely powerful novel of a society in decay. On a planet whose very nature is a mystery a massive decrepit city is pulled along a massive railway track, laying the line down before it as it progresses into the wilderness. The society within toils under an oppressive regime, its structures always on the point of collapse, the lives of its individuals lived in misery. No one knows where they are going, why they are going or what they will find when they get there. The ending of the novel provides one of the most profound twists in SF.