tin can vs underground railway

tin can

noun
  • A metal spaceship or space station. 

  • An inexpensive car. 

  • A motor home or trailer. 

  • A container, usually cylindrical, made out of sheet metal coated with tin, or (by extension) aluminum. 

  • A destroyer. 

underground railway

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

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