#bed.understructure  a foundation of earth or rock supporting a road or railroad track; "the track bed had washed away"
  supertype:  understructure__foundation__base__fundament__foot__groundwork__substructure  lowest support of a structure; "it was built on a base of solid rock"; "he stood at the foot of the tower"
  subtype:  railroad_bed  a bed on which railroad track is laid
  subtype:  roadbed  a bed supporting a road

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