Pony Express vs second class

Pony Express

name
  • A 19th-century mail service delivering messages across North America by means of riders on horseback. 

second class

noun
  • A class of postage with delivery times slower than first class. 

  • A class of travel, inferior to first class. 

  • The second-lowest rank in the Boy Scouts of America, between tenderfoot and first class. 

How often have the words Pony Express and second class occurred in a corpus of books? (source: Google Ngram Viewer )