A "book conveying apparatus" in the tunnel "noiselessly" whisked books back and forth at 600 feet per minute.  Mary Simmerson Cunningham Logan (wife of Civil War era Sen. John Logan) describes the system in her 1901 history of Washington.  With the book tunnel, “a Congressman can get the volumes he desires in less time than it would have taken him when the Library occupied its old quarters in the Capitol itself" writes Logan. " If in the midst of a speech it occurs to a Senator that he needs a certain book or the file of a certain newspaper, he has but to call a page, whisper his wish, and before he has delivered many more sentences, the page returns with the book or file.” 

Central Distributing Desk

The distributing desk in the Jefferson Building's Main Reading Room operated as the central hub in the system.  A request would come in from the Capitol via pneumatic tubes that used to connect the two buildings.  The books would arrive via a separate but similar conveyor system that linked the desk with the dozens of miles of shelving in the north and south stacks. 

The books would descend down to the tunnel room through a dumbwaiter located in the boxy central cabinet.  Then they got placed on the Capitol carrier and whisked away.  The cabinet also housed a little stairway so staff could easily travel between the distributing desk and the tunnel room.