White was originally chosen for the Victoria Line which was completely in
tunnel, as far as this signalling was concerned, and white was not
considered as a problem in this situation. It basically meant that a train
in ATO had authority to pass it whereas a manually operated one did not.
The Jubilee Line simply extended this principle as did the Central Line.

The Vic Line had unilluminated headway boards (viewed by the train lighting)
which simply told the driver that the train had stopped for signalling
purposes due to a train in front. Signals were provided where there was a
need to protect converging routes or inform the driver of diverging routes.
They were also supplied (as headwall corner signals) as starters on every
station. This allowed a degraded mode of operation if a train could not be
driven in ATO. If the train was in ATO the driver could press the start
buttons with a white displayed on the signal, but manual operation had to
wait for the green.

Hindsight is a useful tool, but tripcock testers were supposed to display a
purple/blue light and the white light on the Vic was most likely to be met
under manual driving conditions (as opposed to ATO where the train would
just pass the aspect as required) by ballast/works trains coming on or off
the line - use of an aspect colour to mean different things during a journey
could have caused confusion. Many ATO equipped railways around the world
now use blue and/or maltese cross for this aspect.

BTW - did you know that the original signal aspects on main line railways
were white for clear and red for stop or caution (distants were not well
differentiated in the early days). I do not know when the green aspect
became the standard, but suspect that it was in the early years of the
twentieth century as electric lighting external to the railway became
common. I have an 1896 L&YR rule book which only quotes white and red.

Peter
--
Peter & Elizabeth Corser
Leighton Buzzard, UK
"Boltar" wrote in message
oups.com...

wrote:
It wasn't appropriate to use yellow, as on traditional signalling
systems that would indicate to the driver that he was clear to the next
signal.

They should have chosen something other than white though. If theres
any colour that could be confused at a distance with lots of other
normal
lights nearby its a white. What was wrong with blue or purple for
example?

B2003




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