At the St. Louis airport, the Back 40 isn't part of a ranch. It's the nickname taxi drivers give to a big parking lot just up the road. There they wait for the call to go pick up a fare at the terminal. Spotless white vans sit bumper to bumper in long rows, while the cabbies mill around and talk, or play chess in a small shelter. Waiting can get a little boring. It wasn't hard to get four of them to talk about the ups and downs of making a living in a cab.