First question-
do you mean 2001 as per file name or 2000 as per the heading on your sheets; the numbers seem to make more sense against 2000 so that is what I assumed you meant.
“Determine theoretically, either by calculation or graphically, appropriate signal spacings for the braking characteristics and the intensity of traffic on offer. All calculations and graphs must be shown”
• Don’t waste exam time by re-stating question unnecessarily
• Freight max speed is 100kph; therefore if calculating the constant speed headway at 120kph it is clearly irrelevant; therefore the maximum train length that should be used in calculations is 200m
• Not sure what was the relevance of stating 3 aspect signalling system so early on’ the selection will be driven from the numbers that are being produced.
• Assumptions ok, but should explain more re the contingency allowance made. Given the low non-stop train service, seems a large allowance- obviously makes more sense in the C-D portion where there is the convergence of the two line and also the stopping traffic, but even so needs justification.
• Diagram and formula for best possible 3 aspect headway fine
• Having stated that 3 aspects spaced at 2000m would just meet the requirement with your assumed level of contingency, then I don’t see what the lower half of page 2 and page 3added. You have severe time constraints in the exam- do not waste it! You are basically doing the same calcs again- it should be obvious that increasing spacing to 2000m from 1975m is a minute percentage overbraking so that really is not a consideration- indeed just having 25m tolerance re placing of signals is in reality no flexibility at all. Hence either need to give less contingency or would in practice be forced to use 4 aspects. You were doing ok for the first 1.5 pages; actually thereafter not only were you not gaining marks but actually were showing me that you probably don’t really fully understand- what you wrote was ok, but you were basically doing something that did not need to be done.
• Stopping calcs would have benefitted from diagram- page 5 refers to graph that I have not seen yet
• t2 is basically assuming that braking will not commence until the last possible time and that will then to come to a stand in the platform; you have not considered the cautionary aspect sequence that the driver may have received prior to this (which indeed is what t1 is all about- being on the approach to a signal prior to the station which is still showing a restrictive aspect due to the earlier train still being in the platform or only recently having departed.
• t3 and t4 ok- though obviouslyif the train is starting 25m before the signal then this should have been what was taken into account when considering t2 earlier!
• You have not explained, possibly because you have not understood, the significance of calculating t4 to be the time it takes the train to move beyond the overlap of the signal (which you are implicitly assuming but have not stated to be at the end of the platform). You are stating that the headway time can be calculated as t1=t2+t3+t4 = 148s but you have not explained this and indeed then you state that the minimum time spacing between the trains is 163s- this surely is what is meant by headway time, so you have failed to explain a missing term- (let’s call it t5) that adds those 15secs. I am guessing that you are assuming 3 aspect signalling at 1975m/ 2000m but again you have not stated.
So to actually answer the question that was asked having done both sets of calculations,
• what is the appropriate signal spacing?
• Is it the same for the whole layout area?
Don't think that your answer included a clear statement about these so although the cals were there stopped short of a final conclusion.
do you mean 2001 as per file name or 2000 as per the heading on your sheets; the numbers seem to make more sense against 2000 so that is what I assumed you meant.
“Determine theoretically, either by calculation or graphically, appropriate signal spacings for the braking characteristics and the intensity of traffic on offer. All calculations and graphs must be shown”
• Don’t waste exam time by re-stating question unnecessarily
• Freight max speed is 100kph; therefore if calculating the constant speed headway at 120kph it is clearly irrelevant; therefore the maximum train length that should be used in calculations is 200m
• Not sure what was the relevance of stating 3 aspect signalling system so early on’ the selection will be driven from the numbers that are being produced.
• Assumptions ok, but should explain more re the contingency allowance made. Given the low non-stop train service, seems a large allowance- obviously makes more sense in the C-D portion where there is the convergence of the two line and also the stopping traffic, but even so needs justification.
• Diagram and formula for best possible 3 aspect headway fine
• Having stated that 3 aspects spaced at 2000m would just meet the requirement with your assumed level of contingency, then I don’t see what the lower half of page 2 and page 3added. You have severe time constraints in the exam- do not waste it! You are basically doing the same calcs again- it should be obvious that increasing spacing to 2000m from 1975m is a minute percentage overbraking so that really is not a consideration- indeed just having 25m tolerance re placing of signals is in reality no flexibility at all. Hence either need to give less contingency or would in practice be forced to use 4 aspects. You were doing ok for the first 1.5 pages; actually thereafter not only were you not gaining marks but actually were showing me that you probably don’t really fully understand- what you wrote was ok, but you were basically doing something that did not need to be done.
• Stopping calcs would have benefitted from diagram- page 5 refers to graph that I have not seen yet
• t2 is basically assuming that braking will not commence until the last possible time and that will then to come to a stand in the platform; you have not considered the cautionary aspect sequence that the driver may have received prior to this (which indeed is what t1 is all about- being on the approach to a signal prior to the station which is still showing a restrictive aspect due to the earlier train still being in the platform or only recently having departed.
• t3 and t4 ok- though obviouslyif the train is starting 25m before the signal then this should have been what was taken into account when considering t2 earlier!
• You have not explained, possibly because you have not understood, the significance of calculating t4 to be the time it takes the train to move beyond the overlap of the signal (which you are implicitly assuming but have not stated to be at the end of the platform). You are stating that the headway time can be calculated as t1=t2+t3+t4 = 148s but you have not explained this and indeed then you state that the minimum time spacing between the trains is 163s- this surely is what is meant by headway time, so you have failed to explain a missing term- (let’s call it t5) that adds those 15secs. I am guessing that you are assuming 3 aspect signalling at 1975m/ 2000m but again you have not stated.
So to actually answer the question that was asked having done both sets of calculations,
• what is the appropriate signal spacing?
• Is it the same for the whole layout area?
Don't think that your answer included a clear statement about these so although the cals were there stopped short of a final conclusion.
(06-04-2013, 01:16 PM)merlin89 Wrote: Not so sure about my calculations here. Any feedback appreciated.
PJW