Nick Leverton:
It's a unified system these days. Unfortunately, when the Underground
Group were publicising the Tube back in the first quarter of the last
century, they seem to have omitted to tell people that this word wasn't
to be used for the sub-surface lines...

Actually, not true. It was the Central London Railway (opened 1900)
that first publicized the word "Tube" in a big way,

http://upload.wikimedia.org/wikipedi...%2C_1905.p ng

http://rlv.zcache.com/central_london...65_ihk_400.jpg

and the point was precisely to differentiate the CLR from the
old-fashioned subsurface lines with their tunnels full of smoke.
When the three Yerkes tubes opened in 1906 and 1907, they also used
the word at first, but by that time the subsurface lines were electric
in Central London. When unified maps began to be produced in 1908,
the companies *then* decided to standardize on "Underground".

With no official use of "tube" for decades and no distinction between
steam and electric trains in the tunnels any more, the general public
did not maintain the strict use of the word and...

now you're stuck with it like it or not.

--
Mark Brader, Toronto | "I don't know about your brain,
| but mine is really bossy." -- Laurie Anderson

My text in this article is in the public domain.