The second half of the Jacob's novel contains an extensive analysis of the borders surrounding the city. While not literal, the single use buildings literally keep the city (or parts of the city) in while making a subtle statement to the outside world to steer clear. She considers these boundaries to be dangerous and unhealthy for the life of a city, as the residents feel trapped on all sides and there is no free flow of movement. In short, it creates a dead end in a supposedly vital area.
This reminded me specifically of the wall in Brewster Place. The building had be thriving and full of life and culture and families before the wall went up. After, the people became depressed and defeated and Brewster Place quickly went into a decline. The women, it's new lifesource, felt the effects and began to give up. Maddie moves to Brewster place because she has no other choice. For her and most of the women in the building, Brester really is a dead end both literally and figuratively. The wall not only kept the women in and the world out, but it began to keep them down as well. This physical separation had distastrous consequences and had to be violently removed for the women to be able to breathe.
I was also reminded of a more mental border, but a border no less in Tess' world in Butchers Hill. She describes the supposed hierarchy of the neighborhoods in Baltimore and how the residents are able to feel superior to others based on their location, although only bounded by a name. Certain older residents, she says, will even ask you what side of the water tower you lived on in a specific neighborhood. This is essentially the same thing as the age old question of the railroad tracks to which Jacobs refers. Whether a physical building or wall or a mental boundary and the prejudices of the 'tracks', cities are both bounded from the outside and divided within and as we learn from Naylor's novel, things walled up too long eventually explode.