An older story as it has been at least 15 or more years since I've seen it. I think it was novella length, and was part of a paperback that had several pieces of work by the same author.
I think the plot begins rather abruptly with the main character (male I think) on a train, or some form of transportation similar to a train, travelling to a place he knows of, but has never been. In this future setting, part of the US has been divided off to become a separate entity. It is along one of the coasts, with a mountain range between it an the rest of the US, as the mountains form a partial barrier to air pollution travelling from the US into this area. Although, not being American, I have only vague ideas of US geography, I had the impression that this are would correspond with California. This separated off area is extremely environmentally-conscious and the area separated from the rest of the states in order to create a society that lives in a way that has as little impact on the environment as possible. Cars are not used. Transportation is by this train (which I'm pretty sure does not run on petroleum-based fuel), or walking, or animal powered. Plastic is very rare in this society, and the few items made off it that do exist are very expensive. Manufacturing is limited in this area, as factories create pollution, and therefore I think manufacturing is at the cottage-industry level. There have been some social changes too, but I can't remember clearly what they are, except that I think what we might consider traditional family groupings are not the majority.
My memory is rather vague on the plot, but I think the main character may have been sent from the US on a fact-finding trip, in preparation for political/economic/trade negotiations that are upcoming between this area and the US. I know the traveller is regularly startled at aspects of their daily life that he finds strange. There is one point at which he goes to some sort of market and buys a plastic comb and meets with some disapproval from the person he is with, that he would pay the exhorbitant amount to own plastic. I don't remember how it ends, but I think the main character develops a relationship with one of the citizens of this society and has to decide whether to stay or return to his native US.