railway yard vs piece of ground

railway yard and piece of ground both are nouns.

Word	Noun	Adjective	Verb	Adverb
railway yard	Yes	No	No	No
piece of ground	Yes	No	No	No

As nouns, piece of ground is a hypernym of railway yard; that is, piece of ground is a word with a broader meaning than railway yard:

railway yard: an area having a network of railway tracks and sidings for storage and maintenance of cars and engines
piece of ground: an extended area of land

Other hypernyms of railway yard include parcel, parcel of land, piece of land, tract.

railway yard (noun)	piece of ground (noun)
an area having a network of railway tracks and sidings for storage and maintenance of cars and engines	an extended area of land