Setting of Farenheit 451

The setting of "Fahrenheit 451" is in the future, sometime in the 24^th century, in and around an unspecified city.  Right now it is taking place in the streets of the city at night when pretty much nobody is out and the main character Guy Montag meets a mysterious young girl named Clarisse.  A quote from the book providing details of the setting is,  "He walked out of the Fire Station and walked on the street toward the subway where the silent air-propelled train slid soundlessly down its lubricated flue."  (Page 2) This shows setting because it describes the place around him. what do you think a flue is?