Re: [WA] Zanthus collision 18 Aug

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

Maurie Daly wrote:

> 
> On NSW CTC and most  (all ??) other systems the arrival signals are controlled
> both by the track ccts of both the main and loop , and also by detectors in
> the point machine which senses where the blades of the points are .
> Approach locking is applied directly to the point machine so that it cant be
> changed whilst a train is approaching or a train is over it.
> Any half decent system simply woudnt have let the points be reversed whilst a
> train was sitting in the loop.

Huh? In that case, how do you ever get a train out of the loop, or do a
shunting movement, or do a three-train cross? Probably you didn't really
mean that last sentence in quite the way it came out.

You seem to be constructing a rather circular argument here, viz. if the
area is fully track-circuited, the signals will be controlled by the
track circuits. Obviously usually true, since that is a prime purpose of
the track circuits!

However you have to consider two other situations:

1. Only some parts of the area are track-circuited, e.g. an older-style
NSW automatic crossing loop where the main line is track-circuited
between the respective landmarks but the loop is typically not
track-circuited.

2. There are no track circuits at all, or they cover only the area
immediately around the points.

The Nullarbor is train-order working, and the track circuits (if any)
are extremely localised to the vicinity of the points. There are no
longer-distance track circuits by which approach locking can be
effected. You can't have conditional approach-locking if you don't have
a way of detecting when a train is approaching. 

That is why some systems - but until now, evidently not on the Nullarbor
- have a compulsory timing mechanism so that points cannot be reversed
until a few minutes have elapsed, which gives an opportunity for any
approaching train to be brought to a stand and for second thoughts by
the person who plans to move the points.


Eddie Oliver