From: eoliver@laurel.ocs.mq.edu.au (Eddie Oliver)
Subject: Re: Note on Newport News Wreck
Organization: Macquarie University, Australia.
Date: Thu, 20 Aug 1992 07:54:08 GMT


Several people have kindly sent me e-mail in response to my previous posting
asking about protection for switches in power-signalled areas. There seems to be
sufficient interest for me to expand on what I meant.

Firstly the distinction between power signalling and other forms: I was seeking
to distinguish between automatic and semi-automatic signalling versus manual
varieties (or no signalling at all). One person asked "how can signalling not
be power-operated?" Easily - there are still many purely mechanical
installations in the world, where the signals are operated by mechanical means
and the one-train-in-a-block principle is enforced by non-electrical methods
(or by very primitive methods which only use electricity incidentally).

Maybe I should have added that "power signalling" in British and Australian
usage means ELECTRIC power, not hydraulic power or whatever! 

In Australian power signalling as in the US variety, there are interlocked
areas and fully automatic areas. The discussion was about fully automatic
areas, i.e. where the signals work entirely automatically without supervision
by anyone.

Here any switches (points, in our terminology) in automatic signalling areas
are protected in two ways:

1. for TRAILING switches, special keys are provided which unlock the lever
which operates the switch. These keys are usually Annett keys or similar, i.e.
specially shaped objects several inches long rather than just padlock keys.
The keys are normally retained at suitable stations and are issued to train
crews when they explicitly need to shunt the siding or whatever.

These levers may or may not also be controlled/released by detecting the
occupancy of relevant track circuits.

2. for FACING switches (as in the Newport accident which started all this),
the levers are released by special keys which can only be obtained from
an adjacent electrical releasing device. This electrical device is in
turn released by detection of various conditions in the track circuits,
usually (as I said in the previous posting) involving verification that
a train has been standing on the track circuit immediately in front of
the switch for some specified time interval. This therefore verifies that
the train has been brought to a standstill before the switch can be moved.

Analogous arrangements exist where conflicting movements might be possible
in the midst of automatic signalling areas, for instance if a trailing move
off one track would intersect another track. 

Actually the sort of situation described in the Newport affair seems strange
to us for another reason, i.e. we would not normally have a situation
where a facing switch was a long distance from the protecting signal. We
would usually have a signal immediately adjacent, so that it would not
be possible for someone to throw the switch (by any means) after the train
had passed that signal unless the train had been brought to a standstill.

A typical example of all this is  at Yennora in the Sydney suburban area.
A train wishing to proceed to the Wool Stores siding has to follow the
following procedure:

1. insert a key on the protecting signal to put that signal back to stop.
2. obtain keys from two separate electric releasing devices, one which
verifies that the train has indeed been brought to a standstill and
the other which verifies that the opposing line is clear.
3. put these keys in the respective levers and then move the switches.

It would take a lot of ingenuity for anyone to move the switch in front
of a moving train in that system. You would have to manipulate the track
circuits - admittedly one can always drop a track circuit without a train
being there, but the whole process would take a determined effort by a
knowledgeable person.

Thanks to the people who sent me email, and I hope the above is of some
interest.

Eddie Oliver