Just as transportation networks can create a new megaregion, so can they work to destroy one. Back in 1910, the golden age of manufacturing centralization, the US had a megaregion, namely the core formed by the quadrilateral whose vertices are Boston, Milwaukee, St. Louis, and Baltimore. Outside this quadrilateral, there were almost no large industrial cities, and the main exceptions were either right outside and represented extensions (Kansas City, the Twin Cities) or in California (San Francisco), representing a separate manufacturing network.
The formation of the Northeast megaregion was really a replacement of the old megaregion with a different system. The US used to have a general core area and a periphery. It was never perfect – the North’s dominance of the South was only complete after city regions developed elsewhere, so there was never a total dominance of the manufacturing belt. But it was still far richer than the periphery, and each city had a separate industry to specialize in. In contrast, what we see today is different: there are still core and periphery regions, but instead of one giant core, we have a spikier core, spread relatively evenly nationwide. The Northeast’s four major cities are all core, but there are core cities everywhere, so in a sense we get a core and periphery in each major region of the US. Megaregions occur along clusters of core cities surrounded by their nearby dependencies.
Just as the new, smaller megaregions are bound by high-speed rail or by regular rail and freeway networks, the old one was bound by the emerging freight rail network. Lower transportation costs made it easy to manufacture everything in one place and ship it elsewhere. Once a core-periphery model sets in, it takes a large difference in income to make it worthwhile to start locating factories elsewhere.
I forget which book I got this from (I think The Bottom Billion by Paul Collier), but it took until the postwar period for first-third world income disparity to grow to the point that American companies started offshoring factories. Until then, what is now the first world consistently had higher economic growth than what is now the third world, even absent colonial relationships. Of course within a country the costs of moving factories are lower and so the wealth disparity required to change the core-periphery dynamic is different, but the principle is similar. For decades, there wasn’t much city growth in the South. California grew very quickly, since it had gold and then oil and needed to manufacture its own goods since transportation costs across the Rockies were too high for it to import everything from the East. But, as Jane Jacobs quotes Henry Grady on the situation of the region around Atlanta:
I attended a funeral once in Pickens county in my State. They buried him in the midst of a marble quarry: they cut through solid marble to make his grave; and yet a little tombstone they put above him was from Vermont. They buried him in the heart of a pine forest, and yet the pine coffin was imported from Cincinnati. They buried him within touch of an iron mine, and yet the nails in his coffin and the iron in the shovel that dug his grave were imported from Pittsburg. They buried him by the side of the best sheep-grazing country on the earth, and yet the wool in the coffin bands and the coffin bands themselves were brought from the North. The South didn’t furnish a thing on earth for that funeral but the corpse and the hole in the ground.
Of course, something similar to what happened in the US between about 1910 and the 1960s is happening on a global scale: the dominance of the US, Europe, and Japan is in decline, and third-world economic growth is now consistently above first-world growth.
Paul Krugman suggests that this change in agglomeration patterns comes from further income growth, noting that China today looks a lot like the US of 1910. While he talks about it in terms of city specialization, this is equally true in terms of the location of the core. China’s core today is a contiguous region stretching from Guangdong to just north of Shanghai, plus Beijing. It’s entirely possible that in fifty years it will look different – that the small cities between the megaregions around Shanghai and Guangdong will rust while new cores will develop around large interior cities.