To add to Zaphod comments (which I agree with):
Some bits of it were very good, BUT more than you could do in the exam in a reasonable time. Needs to be more streamlined.
Also very neat; you just won't be able to do like this in exam conditions and it is about time that you were really focussed on exam performance preparation.
List of assumptions could perhaps be pruned:
e.g. since given things like level gradient and metric measurements then needn't state these
Sighting time, overlap length can probably be more easily stated within the calcs themselves
Also don't bother to say "convenient to convert km/h to m/s"; just do it!
Similarly when rounding answers, just do it.
Better to have put the headway diagram with the formula; also could quickly use this to state assumptions of O and S.
No idea why you wasted time working out for 4 aspects; you had shown that 3 aspects very comfortably deliver requirement. Hence this was all wasted time which will gain you nothing whereas you could have done more on the layout that is likely to earn you better marks. In fact it would actually detract from your answer in my view- suggests you don't understand enough not to appreciate that this was a wrong direction to take.
Description just above your 3 aspect diagram seemed very verbose.
You have just worked out that 3 aspects at minimum spacing give headway time of 84sec; since the specified requirement was 150 can simply state that it very comfortably delivers. Move on.
Fast following Stopping.
Your formula would make more sense if the term "x+y" were made the "subject of the expression" since these are the unknowns we are trying to calculate- it didn't immediately look like that.
Also don't like the fact that you just call it a headway distance as the whole point about AD and BD is that the speed isn't constant, so trains that are a constant time apart (which is what headway really means) will not be separated by a constant distance. Be more precise re what you mean.
You are using a bit of a "sneak" way to calculate. It is fair enough as I can see it save time in the exam, but you do need to explain pretty carefully what you are calculating, be very precise re what you are meaning here by "headway distance" .
At the bottom of this page you have decided that there are 48sec of constant speed running which is "spare" within the value dictated by the headway distance fixed by the signal spacing.
It then goes a bit adrift, at least as far as explanation is concerned.
Your method basically is taking the best headway distance (achievable utilising 3 aspects at minimum spacing) between two constant speed trains and determining how much "slack" there is to allow the first to slow down, dwell and accelerate back to speed again without impacting upon following one that thereafter should be running at the specified headway requirement.
So this is the known distance (104s at minimum spacing, up to 150s at maximum spacing) at headway speed. It is being used to derive the sum of (x+y).
The diagram below is useful-although you should more clearly label y axis as Speed and the x axis as time. You are not really showing two separate trains on this though; rather the alternative possibilities for the same train, showing the relative timings between whether it is to go fast or if to stop, The key thing about this is that because the non-stop train is moving at constant rate then you can superimpose distance as applicable in the x direction for THIS train, but be careful not to attribute to the stopping one.
The bottom diagram just leaves me cold.
I can follow the 104sec, non stopping train at a constant speed.
However the obviously distance based diagram with signals overlaps length, trains and platforms has been assigned to the TIME axis not the DISTANCE one. Certainly gives impression is that you have seen some different presentations, not fully understood and therefore got them muddled.
Anyway if we look at the profile of the stopping train
a) there is a portion of 24 sec which should as I understand your previous explanation be at the same constant speed (and thus gradient) as the case of the non-stop (i.e. indistinguishable from it as driver not yet started to brake) going at a different constant speed (if we trust your axes then travelling less distance in more time and thus slower, having changed speed instantaneously).
b) Then a period where there is another step change back to the original speed (judging by the gradient) but just running later in time than it would have been. This is marked 55.6 sec which is supposed to be braking.
c) Then the train suddenly changes from headway speed to infinite speed in an instant; this is marked 30 sec so is supposed obviously to be station dwell so therefore I now know for sure that you should have labelled the y axis as distance and the x as time). So if we assume that instead, it would mean the train goes instantaneously from its headway speed to totally stopped.
d) It then suddenly sets off again at its headway speed.
Hence all this drawing tells me is that you don't understand the significance of the quantities you are portraying. It would be losing you marks rather than gaining them; you might have been given the benefit of the doubt earlier, but this has clarified- and not in your favour!
Looks like you have confused several different diagrams from books etc which sometimes have the time axis horizontal and sometimes vertical, also some diagrams that are velocity /time and others that are distance / time.
The bit on the last page gets back on track a bit, relevant to the applicability of the calcs to the layout to be designed.
You do need to explain the rationale of the 50percent excess signal spacing though.
You also didn't explicitly state that if spaced at 1667m that would still achieve the headway requirement for both non-stop and stopping.
Hope you find the above a helpful criticism
Some bits of it were very good, BUT more than you could do in the exam in a reasonable time. Needs to be more streamlined.
Also very neat; you just won't be able to do like this in exam conditions and it is about time that you were really focussed on exam performance preparation.
List of assumptions could perhaps be pruned:
e.g. since given things like level gradient and metric measurements then needn't state these
Sighting time, overlap length can probably be more easily stated within the calcs themselves
Also don't bother to say "convenient to convert km/h to m/s"; just do it!
Similarly when rounding answers, just do it.
Better to have put the headway diagram with the formula; also could quickly use this to state assumptions of O and S.
No idea why you wasted time working out for 4 aspects; you had shown that 3 aspects very comfortably deliver requirement. Hence this was all wasted time which will gain you nothing whereas you could have done more on the layout that is likely to earn you better marks. In fact it would actually detract from your answer in my view- suggests you don't understand enough not to appreciate that this was a wrong direction to take.
Description just above your 3 aspect diagram seemed very verbose.
You have just worked out that 3 aspects at minimum spacing give headway time of 84sec; since the specified requirement was 150 can simply state that it very comfortably delivers. Move on.
Fast following Stopping.
Your formula would make more sense if the term "x+y" were made the "subject of the expression" since these are the unknowns we are trying to calculate- it didn't immediately look like that.
Also don't like the fact that you just call it a headway distance as the whole point about AD and BD is that the speed isn't constant, so trains that are a constant time apart (which is what headway really means) will not be separated by a constant distance. Be more precise re what you mean.
You are using a bit of a "sneak" way to calculate. It is fair enough as I can see it save time in the exam, but you do need to explain pretty carefully what you are calculating, be very precise re what you are meaning here by "headway distance" .
At the bottom of this page you have decided that there are 48sec of constant speed running which is "spare" within the value dictated by the headway distance fixed by the signal spacing.
It then goes a bit adrift, at least as far as explanation is concerned.
Your method basically is taking the best headway distance (achievable utilising 3 aspects at minimum spacing) between two constant speed trains and determining how much "slack" there is to allow the first to slow down, dwell and accelerate back to speed again without impacting upon following one that thereafter should be running at the specified headway requirement.
So this is the known distance (104s at minimum spacing, up to 150s at maximum spacing) at headway speed. It is being used to derive the sum of (x+y).
The diagram below is useful-although you should more clearly label y axis as Speed and the x axis as time. You are not really showing two separate trains on this though; rather the alternative possibilities for the same train, showing the relative timings between whether it is to go fast or if to stop, The key thing about this is that because the non-stop train is moving at constant rate then you can superimpose distance as applicable in the x direction for THIS train, but be careful not to attribute to the stopping one.
The bottom diagram just leaves me cold.
I can follow the 104sec, non stopping train at a constant speed.
However the obviously distance based diagram with signals overlaps length, trains and platforms has been assigned to the TIME axis not the DISTANCE one. Certainly gives impression is that you have seen some different presentations, not fully understood and therefore got them muddled.
Anyway if we look at the profile of the stopping train
a) there is a portion of 24 sec which should as I understand your previous explanation be at the same constant speed (and thus gradient) as the case of the non-stop (i.e. indistinguishable from it as driver not yet started to brake) going at a different constant speed (if we trust your axes then travelling less distance in more time and thus slower, having changed speed instantaneously).
b) Then a period where there is another step change back to the original speed (judging by the gradient) but just running later in time than it would have been. This is marked 55.6 sec which is supposed to be braking.
c) Then the train suddenly changes from headway speed to infinite speed in an instant; this is marked 30 sec so is supposed obviously to be station dwell so therefore I now know for sure that you should have labelled the y axis as distance and the x as time). So if we assume that instead, it would mean the train goes instantaneously from its headway speed to totally stopped.
d) It then suddenly sets off again at its headway speed.
Hence all this drawing tells me is that you don't understand the significance of the quantities you are portraying. It would be losing you marks rather than gaining them; you might have been given the benefit of the doubt earlier, but this has clarified- and not in your favour!
Looks like you have confused several different diagrams from books etc which sometimes have the time axis horizontal and sometimes vertical, also some diagrams that are velocity /time and others that are distance / time.
The bit on the last page gets back on track a bit, relevant to the applicability of the calcs to the layout to be designed.
You do need to explain the rationale of the 50percent excess signal spacing though.
You also didn't explicitly state that if spaced at 1667m that would still achieve the headway requirement for both non-stop and stopping.
Hope you find the above a helpful criticism
(15-09-2011, 09:15 AM)Zaphod Wrote:(14-09-2011, 10:17 AM)chitrajanarthanan Wrote: Can u please go thorugh this calculation and comment on above calculations???I have only looked through this pretty briefly but my impression is that it's pretty good. A couple of points I noticed:
One of your assumptions states "absolute track circuit block section". I assume that "track circuit block" is meant, as absolute block is something different.
On the second graph on page 5, I think the x-axis is actually depicting time and the y-axis distance. In this case, the line for the stopping train should be below the non-stopping train. They certainly should not converge in the way shown. The figures seem to be right, it's just that the graph needs to be redrawn to make it clear what the figures actually mean.
The sentence "After 189 secs we can pass the fast passenger train" is possibly ambiguous. Does it mean that a fast train can be run 189 seconds after the stopping one? I would possibly reword to avoid the possibility that it means that the two trains should pass one another (which looks like it might be happening based on the graph).
PJW
