David of Broadway wrote

What is the logic to why it does? It has very little in common with
LU, and a lot in common with the rest of Silverlink (which isn't
shown). I remember it starting to appear, but I never understood why
it did (or the rest of BR suburban services didn't).

I think it was first officially shown on the map in 1977 (and I'm sure
someone will correct me if I'm wrong) -- but it hasn't even been
consistently shown since then. At some point in the 80's, it vanished.
And then in the late 80's or early 90's, it returned; around the same
time, the Northern City line (aka Great Northern Electrics) vanished.

Here's an example of the initial unofficial appearance on the map, as
displayed at the Museum Depot:
http://greenberger.no-ip.com/gallery...2_itemId=35525

I note that Camden Road is shown as an interchange with Camden Town - did
this make it onto the official map at any stage?