metro vs underground railway

metro

noun

  • A rapid transit rail transport system, or a train in such systems, generally underground and serving a metropolitan area. 

  • An urban rapid transit rail transport system, normally with lighter track and light trains (i.e. an S-Bahn or similar), or a train in such systems. 

  • A metropolitan area. 

adj

  • metropolitan 

underground railway

noun

  • Synonym of subway: a railway under the ground. 

How often have the words metro and underground railway occurred in a corpus of books? (source: Google Ngram Viewer )