Not as silly as it sounds (although I myself would take a cab instead). There are five terminal buildings in the DFW airport complex, and they are miles apart. It can take as long as an hour during busy times of the day to get from one to the other, if your plane lands at one terminal and your departing flight is in another terminal.
That is true, but they have a little undergound railroad that takes you from one to the other. It is all automated with no driver. Kind of like a horizontal elevator.