Paul Romer on Charter Cities

Paul Romer is mentioned, prominently, in today's Wall Street Journal Review section piece on the "aerotropolis" ("Cities of the Sky", by Greg Lindsay).

Two notable quotes from near the end of the piece,

"If the 20th century was the era of nations, the 21st century is the era of cities."--South Korean President Lee Myung-bak

And the last sentence,

"[The aerotropolis] is at once a new model of urbanism and the newest weapon in the widening competition for wealth and security."

Question:  if we are going to organize our urban areas around transportation systems, why not mid-20th century technology (jet airplanes) rather than mid-19th century technology (railroads)?