I wonder of the US northeast counts as a single megacity.
"A megalopolis (sometimes improperly called a megapolis) or megaregion is typically defined as a chain of roughly adjacent metropolitan areas. The term was used by Patrick Geddes in his 1915 book Cities in Evolution,[1] by Oswald Spengler in his 1918 book, The Decline of the West, and Lewis Mumford in his 1938 book, The Culture of Cities, which described it as the first stage in urban overdevelopment and social decline."
vs megacity: http://en.wikipedia.org/wiki/Megacity
"A megacity is usually defined as a metropolitan area with a total population in excess of ten million people.[1] A megacity can be a single metropolitan area or two or more metropolitan areas that converge. The terms conurbation, metropolis and metroplex are also applied to the latter."
NYC alone is a megacity. The US northeast is a megalopolis.
The Vancouver Metropolitan Area in British Columbia, for example, is six cities, with one transit system. There's also the Greater Vancouver Regional District, which is a megalopolis that consumes a few more adjacent cities, but those aren't linked by transit.
But it's a neat idea to think about transit and accessibility. I'd love to know at which point in the world you can reach the most people within an hour of transit for example, or two hours... This is the advantage in the Pearl River Delta (and probably Tokyo) - there is tons of high-speed rail, subways etc, so you can probably reach about 40 million within 1.5 hr (an hour high speed rail and half an hour on local transit). Whereas it takes me an hour and a half just getting to the airport in Toronto.
And the US Eastern Seaboard was the canonical example, from Northern Virginia/DC up through Boston is about 50M people.