Steve Dulieu:
A "pipe" is once through the tunnel (the main one, the Southgate tunnel
doesn't count), so a typical 4 pipe turn would be one that does pick up
Arnos east and do CK HR123 CK AGw, get off and have grub then
pick up Arnos west and do UX CK AGw, get off and finish. ...

Interesting usage! Do staff on other lines use "pipe" this way?
The Piccadilly isn't the only one with a central tunnel and open-air
sections at both ends (okay, near both ends), but not all are like that --
does this affect the usage?

Is "pipe" also used for the main tunnel itself -- would you say something
like "I was driving east and it failed 15 seconds after I drove into
the pipe" and expect people to know exactly where you were?
--
Mark Brader, Toronto "Information! ... We want information!"
-- The Prisoner

My text in this article is in the public domain.