high road vs main line

high road

noun
  • A course of action which is dignified, honourable, or respectable. 

  • A main road or highway. 

  • Used other than figuratively or idiomatically: see high, road. 

main line

noun
  • A standard sequence of opening moves considered to be best play. 

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