Without significant investment in overhauling its vast rail network, 10 million is probably the rough cap before it reaches a breaking point of some sort. We’re talking easily over a $100 billion worth of expansion and modernization upgrades in new routes, lengthened or widened platforms, adding a fourth track wherever possible, etc. I also think much of the growth (certainly population, but also economic) has to take place in Brooklyn and Queens so that the city isn’t as Manhattan-centric, although this would likely be met with much resistance.
The sky’s (literally) the limit for NYC; it’s always been a city of big thinkers and doers. If there’s one city that has the civic wherewithal to make the seemingly impossible happen, it’s New York.
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