Re: Crossing loop design (r.e. Southern Aurora crash)

[krel4203@netconnect.com.au (Krel )]

On Tue, 9 Feb 1999 22:19:56 +1100, "Notagunzel"
<notagunzel@bigfoot.com> wrote:

>As far as I'm concerned all of this boils down to SPADS & overlaps. Overseas
>(British?) experience is that in most SPADs (Signals Passed at Danger) the
>train pulls up within a fairly short distance. 

Yes, occaisionally you may get a driver roaring past a signal 'bound
for glory', but mostly it is just a slight misjudgment of speed and/or
braking distance. This increases with the pressure to 'get out of the
way" - if the train in the loop has plenty of time there is less
chance of a SPAD; if time is tight the risk increases.

> In a crossing loop,
>therefore, running crosses are relatively safe if there are overlaps beyond
>the Departure Home.  

Assuming that both trains are considerably shorter than the loop.

>On the Western CTC, the approach release is to 'force'
>the Driver down to 40km/h, so if the driver misjudges the braking, the train
>should still pull up within the short (100m?) overlap.  

The problem with this is that with longer trains and the
Australianised Westinghouse air brake system (see another thread)
brake recharges can take more time than required to run into the loop.
This is not such a problem if dyno can be used but if it cannot (or if
it is insufficient) and air has to be used to get down for the "speed
proving" a long train may not be fully recharged before it is
necessary to reapply the air to stop at the end of the loop. 

>On the newer
>suburban loops, Laverton Loop & Lyndbrook etc... the overlap is long, long
>enough to pull up a spark running at line speed when it trips the Departure
>signal. 

To provide enough braking distance from line speed for superfreighters
would require loops to be 4-5km long!!!

>The problem with the NE CTC is that there is no overlap, with
>running crosses under low speed aspects.  This is historical, witness Rock
>and Drome loops (long deceased).  The lack of overlap is exaccerbated if
>drivers enter NE CTC loops at more than 15 km/h. (Is it true it's usually
>closer to 40km/h?) 

The fact that low speed signals do NOT indicate which way the points
lie and do not indicate that the track is clear to the departure
signal also does not help. 

>NSW doesn't have running crosses, with catch points in
>the loop (very British Board of Trade), so crosses are slower than a running
>cross.  

However NSW signals can be cleared as soon as the other train is in
clear and the points have motored. Vic signals seem to take longer to
clear thus the delay is not as much as it would seem.

Cheers

Krel

Just another eccentric crank.