ok, now for a dumb question... What about the word tube for
the London trains? I take it this refers to the tubular
shape of most of the lines/trains?

Correct,

Specifically, it's the shape of the tunnels.

and originally applied to the deep level railways, i.e excluding
the District and the Met, the latter operates the Hammersmith and
City, the Circle and the East London Lines.

Right. The deep-level lines -- which were owned by separate companies
in those days -- might have been at a disadvantage because you had to
wait for a lift (elevator) just to get to the platforms. (Escalators
came later.) But they also had the advantage that their trains were
electric, so the stations weren't full of steam and smoke. So they
marketed themselves as being a New And Improved kind of underground
railway, and the word "tube" was featured in this marketing.

In due course the older lines electrified and got rid of their steam
locomotives, at least on the underground parts, and competition between
lines became irrelevant as they fell under common ownership. By then
the short word "tube" had become established in popular usage, and as
the practical distinctions faded, it spread to the other lines as well.
--
Mark Brader, Toronto | "You keep using that word. I do not think it means
| what you think it means." -- The Princess Bride

My text in this article is in the public domain.