packet boat vs train ferry

packet boat

noun
  • A boat specifically designed to provide transportation to passengers on a regular schedule up and down a river or canal. 

  • A boat that carried mail, cargo, and, in later years, some passengers along a regularly scheduled route up and down a river or canal. 

  • An ocean-going ship chartered by the government to carry the mail and official communications. 

train ferry

noun
  • A ferry used to transport rail vehicles from one railway line to another across a stretch of water. 

How often have the words packet boat and train ferry occurred in a corpus of books? (source: Google Ngram Viewer )