chassis vs trackbed

chassis

noun
  • A base frame, or movable railway, along which the carriage of a mounted gun moves backward and forward. 

  • A woman's buttocks. 

  • The base frame of a motor vehicle. 

  • A frame or housing containing electrical or mechanical equipment, such as on a computer. 

trackbed

noun
  • The layer of gravel or other foundation on which a railway track is laid. 

  • The land on which a railway (especially one that has been closed or dismantled) was built; the roadbed for a railroad. 

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