2008 calculations

[PJW]

Quote:Dear Sir,
I have attempted 2004 mod-2 paper as per indian practice & to my knowledge. Kindly have a look at it & tell me what improvements can i make. I also tried to attach it at IRSEEXAM site but was unable to attach because of the attachment prompt does not open. Waiting for your reply.

Amit Kumar Srivastava

I think it was a high resolution scan and therefore over the 1 MB file size limit. I have printed, commented on it and rescanned and attach here as a pdf

It was extremely good presentation in many ways; however I am afraid that you didn't actually read the question carefully enough as it was a bit different in 2008 than in other years.

a) Determine minimum braking distances for the permissible speeds and braking characteristics of the traffic on offer.
b) Determine graphically, or by calculation, the headway at minimum signal spacing and the
given speed for:
i) A fast passenger train following another fast passenger train; AND
ii) A fast passenger train following a stopping passenger train.
All calculations and graphs must be shown [20 Marks]

For part a) you only calculated for one type of traffic- passenger on the mainline.
You reproduced extremely well an explanation of how to determine the best form of signalling for the layout- but they didn't actually ask for that!

In part b) ii) (that you did first rather confusingly) again you gave a very good presentation but this was for a STOPPING train following another STOPPING train- this was not that which was asked.

In part b) i) you calculated non stopping headway for 4 aspects spaced at 1100m. The question asked for the headway at minimum spacing which is surely the braking distance- there is no hint that you should have utilised 4 aspects. This comment also applies to b) i)

When I first looked at your answer it was without having read the question paper and I then thought it was extremely good. However then I looked at the question paper.
Although you would certainly pick up a lot of marks, you'd also lose a lot for having answered the questions you wanted to answer, not the ones that the exam had set.

As always READ THE QUESTION
DON'T ASSUME

[greensky52]

In part b2, why do you think it means 3 aspect at 2010m spacing?
According to the calculation, 3 aspect can not fit for the requirement. Min signal spacing means the distance between any two signals, or between Red and Green signal? If take 4 aspect and signal spacing as 0.5*BD, isn't it the min spacing?

[PJW]

(16-09-2010 09:26 AM)greensky52 Wrote:  In part b2, why do you think it means 3 aspect at 2010m spacing?
According to the calculation, 3 aspect can not fit for the requirement. Min signal spacing means the distance between any two signals, or between Red and Green signal? If take 4 aspect and signal spacing as 0.5*BD, isn't it the min spacing?

I agree that 3 aspects would not meet the headway requirement specified on the notes of the layout. It depends if you read the question paper itself literally. I think that you could perhaps make a case for interpreting "at minimum spacing" to be "at minimum spacing that not only respects the braking but also the headway requirement" but you did not state that as an assumption.
The wording of the question referred to the categories of traffic and speed but certainly did not mention capacity / headway and therefore I read into the wording is that this was deliberately not to be included within the consideration.

The fact that other bits of your answer seemed to be following the pattern of an approach which was suitable to other years rather than what was requested on this occasion, adds to the view that the examiner is likely to form. That is why you need to be very careful to state your interpretation when a question seems a bit ambiguous- apart from anythhing else it demonstrates that you have read carefully and are trying to answer

[greensky52]

Then even if 3 aspect is avaliable in some layout, but signal spacing of 4 aspect is 0.5*B.D, which must be smaller than 3aspect--B.D. So if the question asked like this one, we should choose 4 aspect to calculate headway?

(16-09-2010 06:01 PM)PJW Wrote:  

(16-09-2010 09:26 AM)greensky52 Wrote:  In part b2, why do you think it means 3 aspect at 2010m spacing?
According to the calculation, 3 aspect can not fit for the requirement. Min signal spacing means the distance between any two signals, or between Red and Green signal? If take 4 aspect and signal spacing as 0.5*BD, isn't it the min spacing?

I agree that 3 aspects would not meet the headway requirement specified on the notes of the layout. It depends if you read the question paper itself literally. I think that you could perhaps make a case for interpreting "at minimum spacing" to be "at minimum spacing that not only respects the braking but also the headway requirement" but you did not state that as an assumption.
The wording of the question referred to the categories of traffic and speed but certainly did not mention capacity / headway and therefore I read into the wording is that this was deliberately not to be included within the consideration.

The fact that other bits of your answer seemed to be following the pattern of an approach which was suitable to other years rather than what was requested on this occasion, adds to the view that the examiner is likely to form. That is why you need to be very careful to state your interpretation when a question seems a bit ambiguous- apart from anythhing else it demonstrates that you have read carefully and are trying to answer

[PJW]

(19-09-2010 08:11 AM)greensky52 Wrote:  Then even if 3 aspect is avaliable in some layout, but signal spacing of 4 aspect is 0.5*B.D, which must be smaller than 3aspect--B.D. So if the question asked like this one, we should choose 4 aspect to calculate headway?

I obviously do not know what the examiners would expect; all I can say is what I would do if a candidate.

My logic is that
part a) says: Determine minimum braking distances for the permissible speeds and braking characteristics of the traffic on offer.
Therefore I would do that for the various speeds / braking rates mentioned.
part b) then says: b) Determine graphically, or by calculation, the headway at minimum signal spacing and the given speed for........
Since this makes no reference to the layout and its headway requirements but only asks what the headway would be at minimum signal spacing, it seems logical to me that 3 aspects are assumed . Potentially could cover yourself by also calculating for 4 aspects spaced at 0.5 BD- I suppose it would not take long to do so and therefore probably worth it. Of course, in the exam always have to decide whether any time spent would be better employed doing smething different that may get more marks per minute......

[greensky52]

Oh, I see... Thanks......

(19-09-2010 08:24 AM)PJW Wrote:  

(19-09-2010 08:11 AM)greensky52 Wrote:  Then even if 3 aspect is avaliable in some layout, but signal spacing of 4 aspect is 0.5*B.D, which must be smaller than 3aspect--B.D. So if the question asked like this one, we should choose 4 aspect to calculate headway?

I obviously do not know what the examiners would expect; all I can say is what I would do if a candidate.

My logic is that
part a) says: Determine minimum braking distances for the permissible speeds and braking characteristics of the traffic on offer.
Therefore I would do that for the various speeds / braking rates mentioned.
part b) then says: b) Determine graphically, or by calculation, the headway at minimum signal spacing and the given speed for........
Since this makes no reference to the layout and its headway requirements but only asks what the headway would be at minimum signal spacing, it seems logical to me that 3 aspects are assumed . Potentially could cover yourself by also calculating for 4 aspects spaced at 0.5 BD- I suppose it would not take long to do so and therefore probably worth it. Of course, in the exam always have to decide whether any time spent would be better employed doing smething different that may get more marks per minute......

[Sid G]

I am inviting guidance and correction on the attached attempt on the calculations part of the 2008 paper; especially with a view to make it achievable within 18 minutes for a novice practising to finish the paper in 90 minutes.

[Jerry1237]

It seems a very reasonable attempt. I've not checked the maths but I do have a couple of comments.

1) equations need to be derived and show what each of the symbols mean.
2) assumptions need to be stated.
3) explination of what is being written is always helpful.

Else, well done.
Jerry

[reuben]

(08-05-2011 07:02 AM)Sid G Wrote:  I am inviting guidance and correction on the attached attempt on the calculations part of the 2008 paper; especially with a view to make it achievable within 18 minutes for a novice practising to finish the paper in 90 minutes.

Feedback

General:

remember to declare your variables
well laid out

Part a

A good approach: work out everything at the start, for all given speeds, because you can't yet predict which outcomes you might need later in the process, also it's much quicker to do them all when you've got the formulae in your head / the conversion factors in the calculator.

Laying out your answers in a tabular form as you have done is also helpful. You could probably save a bit more time by just writing down the generic formula you used for each step, instead of writing it out in full with all the actual figures - the examiner can see that you're just applying the same formula to different input figues.

Part b(i)

I always recommend a very quick sketch diagram here, showing the 2 successive trains and the relevant quantities. this shows that you understand what you're doing, instead of just plucking a formula out of the air; it also helps to declare your variables.

b(ii)

Again, good to draw a diagram here.

The graph must show both trains - if you just show one train, it doesn't demonstrate the time relationship between two trains. also must make clear if it's showing the back or front of each train (you're interested in the back of Train 1 and the front of Train 2.)

t3, i think is intended to relate to the dwell time - if this is the case, this phase of the journey should be truly vertical on the graph, not the vaguely angled line shown

error 1:

The question requires Ht for fast following stopping - if this is the case, you need to allow for Train 1 to accelerate back to full speed rather than just clearing the overlap - it continues to delay the train behind it until it reaches full speed

error 2:

Your calculation for t4 is wrong, it should allow for acceleration rather than travelling at full speed.

error 3

you need to allow for the train length as well as the overlap length; you've only allowed for one of these, I can't tell which because they're both 180m

error 4:

if you're using the "extra time" method, which i recommend because it's very quick, you must account for the time Train1 would have taken anyway, if travelling at full speed instead of stopping. Your calculated values for t2 and t4 are the absolute times, not the extra times compared to non-stopping. Your method partly deals in absolute times, and partly deals in relative times, the two are incompatible.

The actual Textra value should be (2 * (38.89/0.5) )/2 + 30 = 107.8 seconds.

Note:

the first factor of 2 accounts for the extra time during both deceleration and acceleration, you can do this because the acceleration values (0.5) are the same magnitude, even though opposite sign.

The second factor of 2 accounts for the time the train would have taken anyway, if travelling at full speed.

these two factors of 2 cancel each other out, so your answer is co--incidentally about right!

because of error 1, you're not actually interested in the train accelerating through the overlap only, so errors 2 & 3 no longer apply. You've already accounted for train and overlap length at full speed in part b(i), so under the "extra time" method, you don't need to count them again.

Overall, a good answer, most candidates would not attempt b(ii) at all.

hope this helps

[dilip421]

Hello sir,

We have solved 2008 paper by the guidence of our seniors. please go through our attachement & give the corrections.
we have some doubt,
1) Why we required 20% of contigency...?
2) How we are calculating this 20% contigency...?
3) After taking ratio how to decide its 2aspect,3aspect or 4aspect is any written document is provided...?
4) For stopping train why we have to take the total time taken by the stopping train....?
5) The way of explaining in the 2008 layout attachment is correct to write in the exam or we have to show the diagram...?
6) After decideing the aspect how to start the signalling the layout...?

please explain us or forward any solved signalling layout plan. we are got stuckup in that.

Regards,
GDK