J.G. Ballard wrote a short story named “The Concentration City” (you can read the PDF HERE) about a city that has grown to encompass the entire universe… theoretically and practically – whether it is actually is another question.

The protagonist tries to find the edge of the city, to find “free space” – to find an area where he can build and use a flying machine, by hopping a supersonic express sleeper train and riding it west for weeks.

Unfortunately, he discovers that no matter where you go, there you are.

Here in Dallas – which is shaped sort of like a bulls-eye, with concentric rings of highways and radial connectors – all the trains run out of the center and stop. There is no endless loop. They make you get off the train at the end, before it switches track and heads back in.

Otherwise it would be tempting to get on and never exit. Ride the endless electric rails – watch the city go by, circuit after infinite circuit, the commuters come and go. Everything would slide past forever.

Maybe it’s best that you can’t do that.

(click to enlarge)

[…]
“The surgeon hesitated before opening the door. “Look,” he began to explain sympathetically, “you can’t get out of time, can you? Subjectively it’s a plastic dimension, but whatever you do to yourself you’ll never be able to stop that clock”- he pointed to the one on the desk-“or make it run backward. In exactly the same way you can’t get out of the City.”
“The analogy doesn’t hold,” M. said. He gestured at the walls around them and the lights in the streets outside. “All this was built by us. The question nobody can answer is: what was here before we built it?”
“It’s always been here,” the surgeon said. “Not these particular bricks and girders, but others before them. You accept that time has no beginning and no end. The City is as old as time and continuous with it.”
“The first bricks were laid by someone,” M. insisted. “There was the Foundation.”
“A myth. Only the scientists believe in that, and even they don’t try to make too much of it. Most of them privately admit that the Foundation Stone is nothing more than a superstition. We pay it lip service out of convenience, and because it gives us a sense of tradition. Obviously there can’t have been a first brick. If there was, how can you explain who laid it, and even more difficult, where they came from?”
“There must be free space somewhere,” M. said doggedly. “The City must have bounds.”
“Why?” the surgeon asked. “It can’t be floating in the middle of nowhere. Or is that what you’re trying to believe?”
M. sank back limply. “No”
The surgeon watched M silently for a few minutes and paced back to the desk. “This peculiar fixation of yours puzzles me. You’re caught between what the psychiatrists call paradoxical faces. I suppose you haven’t misinterpreted something you’ve heard about the Wall?”
M. looked up. “Which wall?”
The surgeon nodded to himself. “Some advanced opinion maintains that there’s a wall around the City, through which it’s impossible to penetrate. I don’t pretend to understand the theory myself. It’s far too abstract and sophisticated. Anyway I suspect they’ve confused this Wall with the bricked-up black areas you passed through on the Sleeper. I prefer the accepted view that the City stretches out in all direction without limits.””
[…]
—-The Concentration City (1957). James Graham Ballard. The Complete Short Stories