The yellow light blazes through the dank London night, and on it the most comforting word in city life: TAXI. That black box on wheels, the London Taxi, is arguably the greatest urban vehicle ever invented. And it has come to North America. Some 125 are now rolling in cities like New York and Chicago. It's a delicious thought, but here's the rub: A London taxi in London has a cabbie who's spent two years learning his way around (they call it "The Knowledge"). Here, we can only dream of a London taxi driven by a London cabbie. But as my mother always said, Why have economy in fantasy?