7.5 billion and counting: How many humans can the Earth support? Can the Earth support this many people indefinitely? What will happen if we do nothing to manage future population growth and total resource use? These complex questions are ecological, political, ethical – and urgent.

"My proposal, 100 cities of 100 million people, connected by high speed rail, to eliminate transcontinental shipping"

That's not really practical. Firstly, because the reason we largely use ships over air planes for transcontinental shipping is because speed is less important than cost per unit mass. So transcontinental high speed rail is pointless for freight even if you had. If you were going to insist on using railways for that - and I don't see any reason why you would - traditional freight railways would be a better fit. That's why China's new rail line connection to Europe is just a normal speed line and not their famous high speed kind.

Secondly, because cities aren't something you make traditionally. Cities just happen wherever the conditions are right for their creation. There are very few examples of cities raising from nothing sans those for political capitals and those that do exist do so more for the benefit of developers than citizens. You can't re-arrange an entire planet's worth of people. Our largest cities grow naturally. Left alone they'll cannibalise the rural areas and smaller cities around them to feed their need for people. You mentioned Toyko. That's what's happening in Japan. Entire towns are being abandoned because all the young people are moving to the big cities - though that's really a thing in most places it's more pronounced there because while Japan's population is slowly shrinking her largest cities are still growing. You don't need to organise people into cities of 100 million. They'll form on their own as long as there's enough smaller areas in the immediate vicinity to cannibalise or demand for immigration.

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