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

[Eddie Oliver <eoliver@efs.mq.edu.au>]

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! 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).

Eddie