tube vs underground railway

tube

noun
  • The London Underground railway system, originally referred to the lower level lines that ran in tubular tunnels as opposed to the higher ones which ran in rectangular section tunnels. (Often the tube.) 

  • Anything that is hollow and cylindrical in shape. 

  • A television. Compare cathode ray tube and picture tube. 

  • A tin can containing beer. 

  • An approximately cylindrical container, usually with a crimped end and a screw top, used to contain and dispense semiliquid substances. 

  • An idiot. 

  • A wave which pitches forward when breaking, creating a hollow space inside. 

verb
  • To supply with, or enclose in, a tube. 

  • To intubate. 

  • To ride an inner tube. 

underground railway

noun
  • Synonym of subway: a railway under the ground. 

How often have the words tube and underground railway occurred in a corpus of books? (source: Google Ngram Viewer )