G/Y is only found on repeating signals. Repeaters originally had fixed
bottom yellow lights, later some repeaters were made to show reduce to
medium by adding a bottom green light. Thus the two aspects possible were
G/Y or Y/Y Note, repeaters can never show stop. Repeating signals are
pretty rare, they are used when signalling changes from 2 position to 3
position, (eg St Albans on the up, Somerton BG, ). They are also used when
the distant signal is too close to the next home in advance. In this
situation they give the driver more warning of signals ahead than a single
distant would. There have been a few other screwball applications of
repeaters over the years.
The aspect you are looking for is Y/G which is "reduce to medium speed"
Don't get me started on western line approach clearing!!! :-)
ditto for NE SG low speed signals.
Mark.
Some people make art to impress others.
I make art to impress myself.