railbed vs trackbed

railbed

noun
  • The substructure of a railway, underlying the tracks; the roadbed of a railroad. 

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