Help me remember what anime this scene is from!

All I can remember is the intro scene in the first episode. It's some sort of train, and it's an extremely dark and moody scene. The anime is about this strange world that's also kind of citadel shaped. I think it's a prison? And the main character escapes, and meets all these strange people. I watched it as a child, and I just want to revisit it so badly but I can't think of the name. Help?