underground line, specifically an underground cable or wire

line / level / beam

commute to work, go to work

commuting to work

to use / to employ / to make use of, to apply / to spend

(history) a common courtier (a courtier who didn't get access to the imperial palace)

underground; subterranean

use one thing for many purposes / work someone hard

to use one's head

(sciences) ray / (rail transport) line