Definitions:
a long, hollow cylinder of metal, plastic, glass, etc. for holding or transporting something, chiefly liquids or gases

a thing in the form of or resembling a tube.

the underground railway system in london

a sealed container, typically of glass and either evacuated or filled with gas, containing two electrodes between which an electric current can be made to flow.

fit (a person or animal) with a tube to assist breathing, especially after a laryngotomy.

convey in a tube

travel on the underground railway system in london

ride on water on a large inflated inner tube