Maybe I'm going too dark in my interpretation of this, but the environment is always ominous and hellish so maybe that is the point. If you have read any stories about what happens to children when they are kidnaped and turned into "prostitutes", you might agree with me that it seems like the overarching "story" of this series seems to go through those steps.
To summarize the main thing that happens in each chapter:
Ch1 deals with Tengu that are known for their kidnappings, this is even stated in this chapter.
Ch2 they are literally in the world of prostitution, the older girl is mentoring so to speak.
Ch3 she is now being used to recruit another child, the "little sister" character in this case.
Ch4 she ends up taking over the job for two other girls, who are also "sisters".
Becoming a catgirl is then probably symbolic of her being changed by her kidnapers, as this is shown to be something someone is doing to her, the "robot hands". There is probably a lot of other smaller details to go into.
I don't know what this manga is trying to say, it doesn't really seem to portray this evil act as something beautiful, as everywhere she goes things look horrifying, but why base a manga around this theme in such a cryptic way I don't know. It also a bad ending, is it trying to say that without "child to adult" prostitution rings society won't function, as symbolized with the train system that she eventually becomes an important part of. If I'm onto something here that sure puts this author in a wicked light. (Alternatively maybe the train system is just symbolic of the sex trafficking system itself, that she becomes a part of.)
I kinda hope I'm totally wrong, but it seems to fit in a lot of ways.