mattress vs trackbed

mattress

noun
  • A form of retaining wall used to support foundations or an embankment 

  • A pad on which a person can recline and sleep, usually having an inner section of coiled springs covered with foam or other cushioning material then enclosed with cloth fabric. 

verb
  • To cover with a thick layer, like a mattress; to blanket. 

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 mattress and trackbed occurred in a corpus of books? (source: Google Ngram Viewer )