Oh right. It's been a while since I read them. I have Robinson's 2312, which I guess is a sequel to the Mars trilogy. Has a train city running along the dark si

Oh right. It's been a while since I read them. I have Robinson's 2312, which I guess is a sequel to the Mars trilogy. Has a train city running along the dark side on Mercury propelled by the sun on the light side expanding the tracks. I got distracted and never finished the story.