Re: [NSW] Olympic Park/City Railway Signalling Question

[signal_spotter@my-deja.com]

In article <39EAE4A1.C5C09D59@efs.mq.edu.au>,
  Eddie Oliver <eoliver@efs.mq.edu.au> wrote:
> The provision for trains to enter a city station before the previous
> train left DID exist as part of the original Bradfield design. It
worked
> by using the timing tracks along the platform (usually used to
suppress
> the intermediate train stops) in the opposite mode: if a train was
> proved to be DEPARTING quickly enough that it would clear the end of
the
> platform even under emergency braking, then the signal at the arrival
> end of the platform could clear to its most restrictive indication (in
> those days, caution at Museum or St James; low speed at Town Hall or
> Wynyard). The intermediate train stops then worked in their normal
mode,
> so that the entering train could not pass the end of the platform and
> intrude on where the previous train could have stopped.
>
> The facility existed well into recent memory, so Chris's recollection
of
> seeing it happening may well be correct. I only ever saw it happen
once.
> The facility was gradually removed because (a) it was perceived as
> useless in practice, because it was never really used (and even if it
> was, the time saving was minimal), and (b) it was maintenance-
intensive.
> Ensuring the accuracy of the timers for this purpose was obviously far
> more safety-critical than for the normal purpose.
>
> If forced to take a guess, I'd say that the last instance was disabled
> in about the 1970's.
>
> Signal-spotter's posting is incorrect in two respects (sorry, Ian!)
> Firstly, the system above did exist, despite your confident assertion
to
> the contrary!

OK I stand corrected. I wasn't fully aware how the system operated in
the underground and I was basing my response on current practice.

> Secondly, with respect to the more recent signalling of
> the underground, it IS possible for the signal at the entry end of the
> platform to show low speed if only the most limited overlap is
> available; the clearing to caution is a second stage. More or less,
the
> signal will show low speed if the intermediate train stops are active,
> but will go to caution if the train can proceed to the full length of
> the platform with the trips suppressed (although the normal timer will
> operate on the train stop at the platform entry signal).
>
I agree and I think I answered similarly. With 3 ITS's you can have
zero overlap. The signal will show Low Speed if the signal ahead is at
stop and/or there is restricted overlap. The ITS's will be raised and
will lower as the train is timed approaching each one. Basically we're
proving the train is decelerating and therefore reducing it's overlap
accordingly. If the forward route is clear then that will suppress the
ITS's and the signal in rear will step up to a caution.

regards Ian
Signal Design Engineer

> Eddie
>


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