Make it a circle, get rid of the mirror, fill in the center. Bam! You got yourself a city.
Also one of the goals of the project is to reduce the energy needs of the population by making ways shorter. If you spread a city out on a circle, you got a much more complicated and expensive transportation problem than just the linear structure presented here.
In a traditional street network, there are many ways to get from A to B and one link being unavailable is not the end of the world. Forcing everyone onto essentially the same street is guaranteed to cause both high fragility and high congestion, since everyone has to squeeze into the same line.
At the simplest there would be one line connecting from end to end, which should be the fastest and the one with the fewest stops. But then you have smaller lines, travelling in their own tubes, connecting local areas. That way you can travel between any two points relatively fast and avoid congestion. You can build redundancy into this system. But also, it's not a catastrophe on a normally 20 minute trip if there is blocked section and you have to walk a few minutes, take a local connection to bypass the blockage and then go on. Happens to me all the time...
The big benefit in a linear city is that the transport pods never need to turn around. You also don't need to manufacture curved rails or things like that. You can build structures like that in a "2D" city, but it's harder.
In 1D, you can build a linear railway. Cost scales proportionally to length (and hence population) if you assume that the the railway merely needs to get longer as population grows, but this gives a system where ridership on a given rail segment scales linearly with population, which is much worse. Time between two random points also scales linearly with population. The added costs associated with improving performance given the worse scaling seem likely to blow the whole thing up.
Your point about length is true, but about population scale might not, because it's still in 2D. There are still ways to segregate tracks.
I doubt this would work. Cities don't do this for a reason.
Transit lines get built according to transportation needs, not mathematical coverage of the space. You have pockets of housing that need to get to pockets of commerce. You can't practically run transit cars connecting [0,0] and [end, 0].
Beijing is a grid: https://upload.wikimedia.org/wikipedia/commons/0/07/Beijing_...
Shanghai is a grid: https://en.wikipedia.org/wiki/Shanghai_Metro#/media/File:Sha...
Tokyo is grid-like east and south of the Imperial Palace: https://i0.wp.com/transitmap.net/wp-content/uploads/2020/11/...
Taipei has a grid: https://english.metro.taipei/cp.aspx?n=1BE0AF76C79F9A38
The gridded portion of Manhattan has a grid between 14th St and Central Park: https://english.metro.taipei/cp.aspx?n=1BE0AF76C79F9A38. Buses fill out the grid in outer areas like the Bronx: https://new.mta.info/map/5366
Paris is gridded about as much as it can be due to the non-grid road layout: https://www.ratp.fr/sites/default/files/plans-lignes/Plans-e...
Berlin is gridlike: https://sbahn.berlin/en/route-map/
The length is also much less of a concern than in traditional transport systems because of less friction and less need to brake, so most of the energy is for accelerating and decelerating at the stops.
Of course it would blow up if you wanted to build a fully connected city that is twice that length or even longer, but that's not proposed.
Trains crossing perpendicular have to bridge maybe 6m at most. You’d have to bridge the entire length of city for this. And a fire would render any lines above or below unsafe to run until extinguished.
The idea that car-free or very limited car is only possible if you have absurd density is false.
Its totally possible to have nice suburbs that are totally walkable and don't require a car for 90% of the population.
If everyone in Hong Kong was shoved into one train line, it would also be congested.
There are still multi layer travel options available. And some previously unthinkable possibilities (e.g. moving walkways like the kind you see in airports, for short distances within a block – or different colored trains on different levels like ground/underground) are still possible.