closet vs underground railway

closet

noun
  • A secret or hiding place, (particularly) the hiding place in English idioms such as in the closet and skeleton in the closet. 

  • A small room within a house used to store clothing, food, or other household supplies. 

  • One intended for storing clothes or bedclothes. 

  • A state or condition of secrecy, privacy, or obscurity. 

  • An ordinary similar to a bar but half as broad. 

  • The closet can be a scary place for a gay teenager. 

adj
  • closeted, secret (especially with reference to gay people who are in the closet) 

  • Denoting anything kept a secret or private. 

verb
  • To shut away for private discussion. 

  • To put into a private place for a secret interview or interrogation. 

  • To shut up in, or as in, a closet for concealment or confinement. 

underground railway

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

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