Re: Concord derailment

[McFergus@my-dejanews.com]

In article <01bd946a$088acbc0$92056ccb@blakehurst>,
  "Peter Knife" <pijik@knifecutt.fam.aust.com> wrote:
>
> All,
>
> This discussion has triggered a recollection of an old rhyme that was
> apparently drilled into people to help understand the NSW two-light signal
> indications. In part, it went...
>
> 	"and if the yellow's down below, that means reduce to medium slow"
>
> To me, that is an unambiguous indication, regardless of what the driver's
> conditioned expectation might be. It holds the same force as the Y/G
> indication in the VR (US) 'speed signalling' indications, which is a
> warning that the next signal is displaying a medium speed indication. No
> ambiguity.
>

It seems, however, to somewhat negate the advantages of four aspect
signalling. The whole point of four aspect signalling AFAIK is that trains
with greater braking capabilities only need to treat an advanced caution as
just that, and - depending on signal spacing and line speed - not actually
take real action until a caution signal. The following diagrams show a sort
of to scale comparison between BR 3 and 4 aspect signalling. The distance
between the Green and the Red are the same in both cases


Three aspect:

G --------> Y --------> R

            +++++++++++++ braking distance for all trains


Four aspect:

        Y
G ----> Y ----> Y ----> R

  +++++++++++++++++ extended braking distance for freight/fast pax trains 
+++++++++ reduced braking distance for multiple units


If all trains are expected to treat an advanced caution (double yellow for BR
and Brisbane, Green over Yellow for Sydney) as a caution, then why not just
use plain old three aspect signalling? If EMUs can't proceed through an
advanced caution at reasonable speed, then they seem to be a useless aspect!


Rob

Sydney (Australia)

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