Poetically, this universe you've created reminds me of the story of the Berlin metro during the Cold War.
Prior to the division of the city, the metro system spanned the whole city, and was rebuilt to functioning capacity in the aftermath of WW II. Once it became necessary to construct a controlled border between the NATO and Soviet sectors, they eventually erected barriers on the tracks; creating two disconnected transportation graphs.
Except! There were some lines that began and ended in "West Berlin", but needed to pass through a couple of stations in
"East Berlin;" these were allowed to continue operating, but the train conductors were not permitted to stop at the "Easte Berlin" stations under any circumstances. The Geisterbahnhöfe (ghost stations) were patrolled by armed guards. This system persisted from the early 1960s until 1989.
Both sides made improvements to their respective pieces of the U-bahn, to service the local populations. But they never made any changes that would make a reunion impossible (for example, changing the track widths, or signalling technology). When the wall came down, the system united into a fully-functional whole remarkably quickly.
Imagine if the internet partitioned between "east" and "west"; but, in order for the internet backbone to reach India for example, it would first need to pass through Russia or China. I wonder if something similar would happen!
Editorializing, it's fascinating to see how these systems, that are designed to (more or less indiscriminately) connect people, behave when they are suddenly coerced to instead keep people apart in unnatural ways. It never seems sustainable, and the separation seems to revert so easily once the coercion is lifted. It's kind of a hopeful story for humanity, I think.