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underground

noun (TRANSPORT)
 
 
/ˈʌn.də.ɡraʊnd/ US  /-dɚ-/

Definition

the underground A2 [S] (also the tube) UK a railway system in which electric trains travel along passages below ground: the London Underground They went on the underground.Railways and railway lines
(Definition of underground noun (TRANSPORT) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

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