steam generator vs water boiler

steam generator

noun
  • a type of boiler fitted in a diesel locomotive or another rail vehicle, which provided steam heating in steam-heated passenger trains that had previously been hauled by steam locomotives. 

water boiler

noun
  • An electric device which heats water up to boiling temperature. 

How often have the words steam generator and water boiler occurred in a corpus of books? (source: Google Ngram Viewer )