road train vs tractor-trailer

road train

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

tractor-trailer

noun
  • A large truck consisting of an engine and cab section (the tractor) and a cargo-carrying semi-trailer or trailer. 

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