This seems backwards to me. It's Carification and Road Infrastructure that enables such.
The wealth is already there, even if the revolution dented it.
Unless you're referring to the overall Population density of the ASU. In Which case, I would invite you to examine population density on a more local level on the period. I would bet you'll find a mix of low density rural/agricultural areas and higher density urban areas, all of which would be clustered around Railroads or Rivers.
You will find that in the northeast and to a somewhat lesser extent in the Midwest, yes, but in a lot of the Sunbelt even the "cities" are pretty low density at this point in time (and also small). Not to mention all of the peripheral land around cities that can be developed if you only had a better means of getting there...). Many had streetcar systems at this point in time, but largely because there was no better alternative; there's a reason a lot of cities like Houston, Albuquerque, or Atlanta started shrinking their systems starting in the 1920s, long before the post-war era (and apparently at this point in time IOTL half of streetcar systems were bankrupt--I imagine it would be worse here since many of them would have been blown up in the revolution!). It was hard to make rail pay for itself in those areas.
Now, you might say, "but the Sunbelt might not rise!" That's true, but it's unlikely. On the one hand, you have technical advances that make it far easier to live there, on the other the national government is going to want to invest there to alleviate the poverty in many places there (or else why even bother calling itself socialist?), there are a lot of handy natural resources there, and it's now better positioned to trade to the ASU's major partners in the Caribbean and Latin America (since European trade has been suppressed at least somewhat). You might not see so much helter-skelter growth or as much shift from older centers in the Midwest and Northeast, but it's likely to grow, and those areas are just lighter on the old rail infrastructure and are in a position where building for cars
seems to make sense.
We can look back from our perspective and say, "no, no, bad idea!" but that's because we have many decades of experience to observe the negative effects of doing this.
And in truth it probably
does make sense for a lot of places in the United States to simply go for cars and ignore rails, even knowing those impacts. Santa Fe never had a streetcar system for good reason, after all. Of course that's not a big city by any stretch of the imagination, but multiply that by hundreds or thousands of places in a similar boat...it's still a lot of cars and a lot of places that are car-oriented, where the train is really only for getting to some other bigger city.
Especially as the Car is, well, Individualistic and Capitalist. You don't just need Cars. You need Cars; companies invested into selling more of them, damn the consequences; the advertising to sell them en-masse, and a population willing to go along with it all. Your need for a car is inversely proportional to the quality of local public transport. Your desire for a car is proportional to how prevelent the cultural ideas that a Car means Success and Status are.
And why do you think that being socialist would prevent any of this? The auto worker unions certainly have every reason to try to persuade the public to buy cars and leave the railroads, even if the railroad unions would rather they didn't (and anyway the railroad workers probably won't care for a while since cars aren't really competing with them for now). So do the oil worker unions. The national government has a strategic reason to encourage a healthy automobile manufacturing sector (to wit, as I previously noted, the defense uses of motor vehicle manufacturing capacity). Local governments in at least some places have reason to think that switching to more car-focused infrastructure is better than previous modes, mostly smaller cities and places that don't have large rail networks already. The idea of having a nice, shiny new home on your own plot of land with a form of transportation that can keep up with a locomotive and isn't beholden to schedules is hardly difficult to sell as a socialist triumph either, instead of a capitalist one--you're providing a standard of living better than that a king once had to the ordinary worker, after all.
You see "capitalistic," I see chimeric; something that can be spun to fit whatever system it is in, at least at this point in time. Individualistic, sure, but American culture has always had a strain of the individualist and there is no way that any mere political revolution is going to get rid of that without many decades of cultural shift.
But I don't see anything like the Interstate System. Train Infrastructure is just more efficient. No need for Cross-Country Trucking when you can just... Use Trucks for the Last-Mile both ends.
There is
absolutely going to be an Interstate system. There's a reason literally
every developed country has built substantial motorway systems, including the Soviet Union while it was
still the Soviet Union. Trucks are far more efficient than trains for certain cargoes, particularly lightweight, time-sensitive ones, since they don't require large marshaling yards and aren't beholden to the schedules of the railroads (and some cargoes are just not economical to ship for the railroads). Plus, they're really useful for military purposes, and the ASU is obviously going to care about that. This holds true over long distances, too. What you might avoid is Interstates going straight into and through downtown areas and bulldozing bunches of them, at least beyond a few early attempts. That's a huge deal, and has a big impact on car-oriented design, but still doesn't get rid of the Interstates.