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2008 mainline layout attempt
#11
Jerry1237 Wrote:Can I ask a naive question. Why do we take de-acceleration of a train to be 0.5m/s/s?

Other than because the examiners tell us to? Do you mean "why do the examiners pick 0.5m/s/s for deceleration?" or "what is the basis of 0.5/m/s/s?"

For the former, I guess it is because it has to be something and, while still requiring you to be able to calculate things, it does not make the sums especially hard.

For the latter, yes, a modern passenger train may achieve 9% or in some cases 12% g braking giving 0.8 - 1.1 m/s/s, but then you'd have to apply a different rate for the freight.

What did you want to use?
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#12
Peter Wrote:
Jerry1237 Wrote:Can I ask a naive question. Why do we take de-acceleration of a train to be 0.5m/s/s?

Other than because the examiners tell us to? Do you mean "why do the examiners pick 0.5m/s/s for deceleration?" or "what is the basis of 0.5/m/s/s?"

For the former, I guess it is because it has to be something and, while still requiring you to be able to calculate things, it does not make the sums especially hard.

For the latter, yes, a modern passenger train may achieve 9% or in some cases 12% g braking giving 0.8 - 1.1 m/s/s, but then you'd have to apply a different rate for the freight.

What did you want to use?

I do think that it is rather unrealistic for the exam seemingly always assuming acceleration = braking rate, freight = passenger, service = emergency. The metro layout tends to have different rates for accleration, service and emergency braking.
0.5m/s/s is of course particularly handy when working out braking distance as division by "2a" is very simple! trying to recall whether we were allowed calculators in my day of doing the exam; I am not sure that we were (though they did exist by then but only just). To put things in context, there was one electronic calculator in my school which only those in the 6th form could use kept locked away in a room in the science labs; it was the size of a two desktop PCs stacked vertically, used wirewound neon tubes for display and used "reverse Polish" notation: [Enter first number] xxxx, [Enter first number] yyyy, [+ or - or / or *] and that was it!
Probably IRSE exam set in stone in that era!

Probably the truth is that the examiners find that many struggle with the calculations as it is so they daren't make things harder.
PJW
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#13
Okay, so effectively it is a legacy figure. That makes sense.

Some work I am involved in is trying to derive brake rates for freight trains of variable lengths/loads. The figures required, although they are the second line of defense, still are required to be realistic enough to stop a train, with a "normal" level of adhesion, in a predicatable yet operable distance. Hence, my hidden question was where is the engineering justification behind the value or was it a reasonable assumption from days of old. The answer appears to be the latter.

When we have some typical freight figures, I'll publish them here. They will make interesting reading based on theory backed up with practical testing.
Le coureur
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#14
(20-02-2009, 02:46 PM)Jerry1237 Wrote: Okay, so effectively it is a legacy figure. That makes sense.

Some work I am involved in is trying to derive brake rates for freight trains of variable lengths/loads. The figures required, although they are the second line of defense, still are required to be realistic enough to stop a train, with a "normal" level of adhesion, in a predicatable yet operable distance. Hence, my hidden question was where is the engineering justification behind the value or was it a reasonable assumption from days of old. The answer appears to be the latter.

When we have some typical freight figures, I'll publish them here. They will make interesting reading based on theory backed up with practical testing.

Also interesting from an "effectiveness of TPWS" perspective. NR's policy is to asssume a 12%g emergency brake rate after intervention with only a very nominal brake build-up delay. Problem is we know that many passenger trains don't really make it and freight certainly don't. We do "look at" optimising the actual placements for 9%g but not proving more equipment than we'd need for 12%g but deploying it to more closely reflect reality. One of the issue is that the "Emergency" brake rate of certain trains (notably the red tiliting ones...) is significantly LESS than their usual braking rate where "clever" braking such as regenerative is employed rather than relying on friction brakes.

We also assess the risk based upon 4.5%g and determine whether the fitment previously determined is ALARP. However it is actually the brake build up more than the actual final brake rate that is most important from intervention at TSS (though not from OSS). TPWS was of course a "quick and easy cost-effective" solution to give GOOD protection in the MAJORITY of circumstances; it has proved itself great for that but could never be fully effective train protection.

Hence I'd be interested in seeing them but you might wish to ensure official clearance to publish first as they could just open up a whole new "can of worms" between TOCs/ FOCs and NR; at present the lid is on with only a few tails hanging out.........
PJW
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#15
After some work on Friday, it appears as if the rate of 0.5m/s/s can be obtained from the curves on GK/RT0034 App A3 (A1 being appropriate but currently under review!). TOPS actually provides the train's speed to the crew based upon available brake force.

Bizarrely, and contrary to the common held view, the shorter the train the less able it is to slow down! However, the norm is headway is a theoretical calculation which is almost impossible to deliver in the real world due to effects such as dwell times, defensive driving and human factors.

I can provide some data on brake build up times if that would help. RGSOnline has the relevant vehicle standards as GM/RT204n (n is 0 to 6).
Le coureur
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#16
please find attached two files, the attemptes for the 2008 mainline layout and the calcs;

I was looking for some general feedback,

thanks,

Priyank


Attached Files
.pdf   2008 Mainline layout Calcs1.pdf (Size: 349.81 KB / Downloads: 189)
.pdf   2008 Mainline layout attempt1.pdf (Size: 582.59 KB / Downloads: 267)
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#17
(18-09-2009, 04:29 PM)priyman Wrote: please find attached two files, the attemptes for the 2008 mainline layout and the calcs;

I was looking for some general feedback,

thanks,

Priyank

I am working this whole weekend- I might get a chance on Monday or Tuesday evening. However if anyone else wishes to comment earlier ....
PJW
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#18
(19-09-2009, 09:38 AM)PJW Wrote:
(18-09-2009, 04:29 PM)priyman Wrote: please find attached two files, the attemptes for the 2008 mainline layout and the calcs;

I was looking for some general feedback,

thanks,

Priyank

I am working this whole weekend- I might get a chance on Monday or Tuesday evening. However if anyone else wishes to comment earlier ....

OK printed, though twhether I actually have time to look at is questionable. However I have looked at calcs; see attached


Attached Files
.pdf   2008 layout calcs PP.pdf (Size: 773.53 KB / Downloads: 186)
PJW
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#19
(21-09-2009, 05:32 PM)PJW Wrote:
(19-09-2009, 09:38 AM)PJW Wrote:
(18-09-2009, 04:29 PM)priyman Wrote: please find attached two files, the attemptes for the 2008 mainline layout and the calcs;

I was looking for some general feedback,

thanks,

Priyank

I am working this whole weekend- I might get a chance on Monday or Tuesday evening. However if anyone else wishes to comment earlier ....

OK printed, though twhether I actually have time to look at is questionable. However I have looked at calcs; see attached

I've had a look at the layout. Will try to type in response tomorrow.

Peter
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#20
(21-09-2009, 08:35 PM)Peter Wrote:
(21-09-2009, 05:32 PM)PJW Wrote:
(19-09-2009, 09:38 AM)PJW Wrote:
(18-09-2009, 04:29 PM)priyman Wrote: please find attached two files, the attemptes for the 2008 mainline layout and the calcs;

I was looking for some general feedback,

thanks,

Priyank

I am working this whole weekend- I might get a chance on Monday or Tuesday evening. However if anyone else wishes to comment earlier ....

OK printed, though twhether I actually have time to look at is questionable. However I have looked at calcs; see attached

I've had a look at the layout. Will try to type in response tomorrow.

Peter

Actually I had just started looking whe I noticed that Peter has reviewed and will be commenting. Hence I'll leave Peter to write his critique and I'll drop onto doing something else.
However although I had yet to start any form of detailed check and was only looking at "artistic impression" I have to say there is quite a lot of red ink over the plan already.

It features quite a lot of the common howlers (DO READ EXAMINERS COMMENTS AT PREVIOUS YEAR'S EXAM REVIEWS AND MAKE SURE YOU AVOID THE HOWLERS):
# very odd ideas about trap points,
# freight yard with lts of signals and motor points but no track circuiting or explanation how it works safely,
# "facing" LOS,
# preset shunt provided in the middle of nowhere fo no good reason,
# I am yet to understand the supposed method of working on the branch (but I wouldn't travel on it!)
# 552/554 placed on gantry for no good reason and prevents the use of stabling sidings by two 180m trains,
# overlaps just measured to be 180m beyond signal and placed without thought within S&C
# point numbering seems to carelessly miss some, switch diamond looks like double slips
# lack of bufferstop lamps and TCI
# inability to be able to make all operational moves specified

Can't say that I am too impressed so far; impression given is that candidate half knows quite lot of stuff but really hasn't prepared well enough and is somewhat confused. Given that the exam is less than a fortnight away there's a lot to learn in the remaining time.
PJW
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