On Wed, 22 Mar 2006 15:53:11 -0600, "Tristán White"
wrote:

Incidentally, anyone know why the NLL is the only railway to
consistently, since the 1970s, to appear on every tube map
(notwithstanding the occasional appearence of the WAGN and sometimes the
Thameslink on more detailed tube maps?)

It's not the only one - the Waterloo & City did too, while it was a BR
line.

(Not that that answers your question. I suspect it's to do with the
fact that the NLL runs almost entirely through LU territory, and has
lots of interchanges with the various Underground lines.)