Anne Ogborn wrote:

> Interlocking signals are at junctions, and signify the position of the
> various track switches and whether various tracks are cleared through
> the junction.

  Mmm.  The twin-three-aspect signals used in Metropolitan Sydney with
which I grew up (befriended the local (Tempe) stationmaster and got to
look over the signal house once - a terrific buzz for a young devotee of
logic), use amber in the "home" set to indicate *either* "stop" in the
next block *or* a switch.

  Amber in both "home" and "distant" uniquely indicated a double switch
(or maybe just a single switch - it's a while since I lived there now!),
not to be confused with green-amber which simply indicated green-red in
the next block.

  Then there was the 1/3 size (irised) green "calling-on" aspect which
permitted a train just to clear the station or such.  7 aspects - that
was my favourite.

  Sigh.  If I ever get back into modelling, it will be in "N" or "TT"
gauge.  Grain-of-wheat bulbs are passˇ nowadays, SMD LEDs rule.  You can
even make the *back* of the aspect look true-to-life!
-- 
  Cheers,
        Paul B.