road train vs semi

road train

noun
  • A transport concept consisting of a conventional prime mover truck pulling two or more trailers. 

semi

noun
  • A semi-trailer; a tractor-trailer; an eighteen-wheeler; an artic. 

  • A semifinal. 

  • A semi-detached house. 

  • A partial erection of the penis. 

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