Re: Indian Pacific hits freight train - Who's the Responsible Investigating body?

["Notagunzel" <notagunzel@bigfoot.com>]

Maurie Daly <mauried@commslab.gov.au> wrote in message
mauried.411.37C5C757@commslab.gov.au">news:mauried.411.37C5C757@commslab.gov.au...

> Noeone will even know what really happened , but one question that is
left
> begging is that if we accept that sand can insulate wheels from a track
cct it
> doesnt in any way alter what the function of the track cct is,ie if a
train
> goes into a track section and doesnt come out the other end , then its
still
> in there , no matter what the continuity between the rails is ,and the
signals
> should reflect the last known state.
> Since this accident has the interlocking been modified to operate on a
train
> in / train out basis , ie we dont now rely on continuity between the
rails .
> If as I suspect is the case , no modifications have been made then we can
take
> for granted that sand wasnt the original problem as to admit that this is
the
> case and then do nothing about correcting the problem (AFAIK locos still
carry
> sand and use it on the Cowan Bank) would be negligence in the extreme.

Having an insulator between the wheel and the rail is usually fatal for any
form of track circuit.

Reading the above I think there is some confusion as to how a track circuit
operates.  In it's most basic form (Steady DC), the sections are broken up
by insulated joints.  At one end of the section, the voltage is fed to the
track, and at the other end the track relay is between the rails.  So the
circuit goes:
B2 (positive side of battery), feed resistor, one leg of the track circuit
(rail), relay, other leg of the track circuit (rail), N2 (negative side of
battery).

So the relay picks up when the track is unoccupied, and drops away when a
train enters the section.  This is because the trains wheels act as a short
circuit.  The relay will also drop, simulating the presence of a train, if
you use a piece of wire to short the rails, (kids living near level
crossings find this out annoyingly regularly), or if the bonding between
consecutive rails breaks, or the rail itself breaks, or a track lead is
broken, or if the ballast resistance falls due to heavy rain, or if the
mains power fails and the track battery goes flat.

So if a train is either lightweight (Victorian Sprinter), or is sanding
VERY heavily (alleged to be 3801), the trainshunt (resistance of the axles
of the train) can rise until its not enough to shunt enough current to drop
the relay (Ohms law etc...).

Therefore, a train drops the relay, and if it drops a lot of sand or goes
high resistive, it 'disappears'.  This is what I understand to have
happened on Cowan Bank, and with the Sprinters.

European railways I believe have an arrangement where the track circuit
will not pick up until the next track in sequence is down, apparently due
to European ideas on proving relay operation.  This has the effect of
ensuring that a train 'dissapearing' off a track circuit doesn't actually
cause false operation of signals, points, level crossings, etc...  I
remember reading that one of the recent resignallings somewhere in NSW will
actually generate some sort of alarm whenever 'non sequentual' operation
occurs, this may be what you are alluding to.

Why can't this 'sequential testing' be used in Australia, I hear you ask?
Well British, American and Australian railways have always used a different
philosophy when designing anything vital within a signalling system, i.e.,
if you open a contact with a vital relay in a vital circuit, you can
usually assume that the relay will drop, whereas the Europeans almost
always want to prove that the relay actually dropped.  When applied to
track circuits, this philosophy goes that if you design a track circuit,
then it must drop whenever a train enter the section.  If we started
proving sequential operation, it would create problems if a track circuit
drops, then it needs the next track down to pick up, which leaves problems
if there is an intermittent fault, instead of self clearing, it would then
get stuck down.  This then raises several problems, excessively unreliable
tracks then mean more caution orders, hand throwing of points, continuously
operating level crossings, all of which mean a reduction in safety anyway.
Why bother, when you can put the effort into ensuring that the tracks work
properly in the first place(i.e. avoid crummy low voltage circuits)?

--
Mr Notagunzel.
Rail Transportation Connoisseur.
notagunzel@bigfoot.com
(Waiting for the next move at http://www.bigfoot.com/~notagunzel)