"Peter Masson" wrote in message
...

Presumably the signal itself uses an LED display, so that the upper head
can
display green or yellow, and the lower head yellow or red, and the signal
as
a whole can display all four aspects R, Y, YY, and G.

I think I read somewhere that both heads can display red, but that normally
the lower one is used for red. The upper head can be used in emergency to
display red if the lower one has failed for any reason. BICBW.

Regards

Jonathan

On Mar 31, 5:59 pm, "Jonathan Morton"
wrote:

"Peter Masson" wrote in message

Presumably the signal itself uses an LED display, so that
the upper head can display green or yellow, and the lower
head yellow or red, and the signal as a whole can display
all four aspects R, Y, YY, and G.

I think I read somewhere that both heads can display red,
but that normally the lower one is used for red. The upper
head can be used in emergency to display red if the lower
one has failed for any reason. BICBW.

That is wrong. The signals used on the Manchester South scheme can do
that, but they aren't LED type.

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