Synonyms for Underground

Tube
el electric elevated feeder flier main line metro	monorail rail railroad railway roadbed roadway sidetrack	siding streetcar line subway switchback terminal terminus track	tram trestle trunk trunk line tube turnout underground

Surreptitious
backstairs clandestine deceitful feline furtive	privy quiet shifty skulking	slinky sly sneaking sneaky	surreptitious undercover underground unobtrusive

Local
airfreight city confined el electric flier freighter funicular	interurban limited local metro monorail municipal peculiar	precise private restricted rolling stock shuttle special specific	streamliner subway town tram tube underground village

Secret
abstruse ambiguous arcane beclouded buried censored cheap classified closed clouded concealed covert cryptic enigmatic equivocal	esoteric extraterrestrial guarded hermetic hidden incommunicado inner latent mysterious obscured occult personal potential private privy	profound recondite restricted secluded secret sequestered shrouded smothered stifled suppressed surreptitious top secret ulterior under wraps undercover	underground underhanded underlying undisclosable undisclosed unearthly unexpressed unknown unrevealed unspoken untold unutterable unuttered