Commonly used as a nickname to describe the London Underground, the “Tube” is an obvious reference to shape of the tunnels themselves.
The word is believed to have been first popularised around 1890. Underground lines had previously been constructed using the ‘cut and cover’ method – that is, digging out the trench for the Underground line, lining the tunnel with iron and them covering it.
Thanks to its depth (due to the fact it had to pass under the Thames), City and South London Railway’s line from King William Street (a now disused station) to Stockwell was the first to be created by boring a tunnel through the earth and then lining it to create a tube. The line, a forerunner of what is now the Northern Line, opened in December, 1890. It was the first true ‘tube’ of the Underground system.
The construction of the line was followed in 1900 by the opening of the Central London Railway’s line from Shepherd’s Bush to Bank (now part of the Central Line) which was given the nickname the “Twopenny Tube”.
The name stayed and was soon applied to the entire network of Underground lines (and interestingly, it wasn’t until 1908 that the word “Underground” first appeared in stations).