05-07-2012, 09:06 PM
RE: Appendix W, Activity 1A - Q8
Savitha,
Yes you do seem to have understood this quite well.
As you stated, the signalling must be safe for the train type needing the longest braking, so minimum spacing 1013m.
There are then 3 possible constraints for the maximum spacing, all of which must be satisfied-
a) the need to meet the headway specified for passenger trains during the day,
b) the need to satisfy the headway specified for freight trains during the night,
c) an acceptable amount of over-braking, typically considered to be 133% of the actual braking, so lets say 1350m.
It is the lowest of these figures which is the determining maximum spacing.
When you did your sums, you used the technical headway figures rather than the figures with the contingency allowance built into them, so I'd therefore say you have overestimated the maximum in each case. You might indeed also have chosen to give the freight a smaller (or indeed nil) contingency as you could well argue that the frequency is lower and the precise timing of freight less critical.
However these are small points; you showed that you needed to consider all the possible constraints and determine which of these would turn out to be the most onerous and then use that figure. Most important you did not attempt to suggest the signal spacing would be different in the night-time from the day-time; you'd be surprised how many state that nonsense (I envisage an army of technicians re signalling twice per 24 hour period!).
If I have understood your concern correctly, I think you are worrying that the 133% will not be satisfied for freight trains when the signals are spaced to be suitable for passenger trains. Actually this is not something to be concerned about. The reason why over-braking is a concern at all is because we want to make sure that drivers do feel it appropriate to brake when passing a caution aspect, rather than them become accustomed to signal spacings vastly bigger and thus believing that braking is inappropriate. The problem is that if they leave braking for a bit, it may slip their mind if there is some other distraction such as needing to sound horn because of trackside workers and then the next they know they are nearly at the next signal still going at full speed.
However 133% is not a precise figure of acceptable / unacceptable; just a guideline within the continuum of gradually increasing risk. Indeed consistency of signal spacing from one section to another is also a significant consideration, and variations in speed profile and gradient profile will mean that the degree of over-braking will vary; the important thing is not for any one section (unless it is so ridiculously different that it becomes "known" to the drivers as a special case) but it is about accustomisation and thus general expectation.
It just isn't possible to fulfill all the "nice to haves" for a variety of trains with different characteristics. Actually freight is driven rather more defensively than passenger trains, for a variety of reasons:
1. Tends not to be as fast, nor so absolutely critical to timetable so precisely. Yes it has to share tracks with other services, yes time punctuality is more important now than historically but it isn't like having a lot f people on board all looking at their watch anxiously hoping to make a connection with another service- freight trains generally go from origin of journey of the goods direct to their destination, whereas passengers typically have a journey involving several trains and perhaps bus and if they just miss their connection by seconds they may have a long wait.
2. Braking is not so good and acceleration far poorer because of power to weight ratio. Also uses considerably more energy to speed up and a lot of brake wear and heat dissipation to slow down (whereas passenger stock often now uses braking that does not involve only friction against brake pads and increasingly can regenerate power to put into the traction system of an electrified railway.
3. Braking also takes a significant time to fully apply and also to cancel as it uses air pipe pressure along the length of a long train rather than generally electrical control over a typically much shorter passenger train. Therefore changing between braking and accelerating and then braking again isn't something to be undertaken if it can be avoided.
This all means that drivers of freight typically brake early to reduce their speed significantly, then cease active braking but coast at a very slowly decreasing speed until they need to brake again if they actually need to stop. The last thing they want to do is stop unnecessarily as that wastes energy and indeed the time needed to restart, so it is to their benefit to take as long as they can when approaching a signal at red to give it the best chance of clearing whilst they are still far enough away that they are still moving at a significant speed when it clears. Therefore over-braking is less of a concern for them than passenger train drivers for whom a few seconds of running time is more significant and therefore more prone to deciding to avoid braking sooner than they feel is really necessary.
This is a question that often catches people out and is worse than you are likely to encounter in the exam, but useful to find who is really understanding and who just puts numbers into formulae. Seems to me that you are in the first category.
(Tuesday, 3rd July, 2012 05:16)savitha kandasamy Wrote:
(Monday, 25th June, 2012 12:43)PJW Wrote: Generally seem to have made a good attempt.
You correctly determined that it is the passenger train rather than the freight train which is the more onerous constraint regarding the minimum signal spacing for braking, but it would have been good to have stated this explicitly.
Conversely you do not seem to have considered whether the capacity requirement needed for the freight trains at night gives rise to a more onerous constraint on the maximum spacing than that imposed by the passenger train required headway, or indeed the maximum amount of over-braking considered tolerable.
Also in the exam don't just quote the formula re Headway, but explain using a diagram its derivation.
As per the operational requirements for passenger trains,
Minimum Signal spacing = 0.5*BD1 = 0.5*2025 =1013m
Maximum Signal spacing, d =((Ht4*Vh)-(S+O+L))/3
= ((120*36)-640)/3
=1227m
As per the operational requirements for freight trains,
Minimum Signal spacing = 0.5*BD2 = 0.5*1458 =729m
Maximum Signal spacing, d =((Ht4*Vf)-(S+O+L))/3
= ((180*27)-850)/3
=1337m
The longest braking distance needs to be considered to ensure safety. So the minimum signal spacing is 1013m.
For Maximum signal spacing, if we considered freight train headway requirements as more onerous , we can place the signals at 1337m. This is within 33% of BD. But the headway requirements for passenger trains cannot be achieved with this maximum signal spacing. Also I am not sure how the braking distance requirement for freight will fit with the maximum signal spacing (if we choose either 1227m or 1337m). Kindly suggest me how to justify for the operational requirements for freight trains.
savitha
Savitha,
Yes you do seem to have understood this quite well.
As you stated, the signalling must be safe for the train type needing the longest braking, so minimum spacing 1013m.
There are then 3 possible constraints for the maximum spacing, all of which must be satisfied-
a) the need to meet the headway specified for passenger trains during the day,
b) the need to satisfy the headway specified for freight trains during the night,
c) an acceptable amount of over-braking, typically considered to be 133% of the actual braking, so lets say 1350m.
It is the lowest of these figures which is the determining maximum spacing.
When you did your sums, you used the technical headway figures rather than the figures with the contingency allowance built into them, so I'd therefore say you have overestimated the maximum in each case. You might indeed also have chosen to give the freight a smaller (or indeed nil) contingency as you could well argue that the frequency is lower and the precise timing of freight less critical.
However these are small points; you showed that you needed to consider all the possible constraints and determine which of these would turn out to be the most onerous and then use that figure. Most important you did not attempt to suggest the signal spacing would be different in the night-time from the day-time; you'd be surprised how many state that nonsense (I envisage an army of technicians re signalling twice per 24 hour period!).
If I have understood your concern correctly, I think you are worrying that the 133% will not be satisfied for freight trains when the signals are spaced to be suitable for passenger trains. Actually this is not something to be concerned about. The reason why over-braking is a concern at all is because we want to make sure that drivers do feel it appropriate to brake when passing a caution aspect, rather than them become accustomed to signal spacings vastly bigger and thus believing that braking is inappropriate. The problem is that if they leave braking for a bit, it may slip their mind if there is some other distraction such as needing to sound horn because of trackside workers and then the next they know they are nearly at the next signal still going at full speed.
However 133% is not a precise figure of acceptable / unacceptable; just a guideline within the continuum of gradually increasing risk. Indeed consistency of signal spacing from one section to another is also a significant consideration, and variations in speed profile and gradient profile will mean that the degree of over-braking will vary; the important thing is not for any one section (unless it is so ridiculously different that it becomes "known" to the drivers as a special case) but it is about accustomisation and thus general expectation.
It just isn't possible to fulfill all the "nice to haves" for a variety of trains with different characteristics. Actually freight is driven rather more defensively than passenger trains, for a variety of reasons:
1. Tends not to be as fast, nor so absolutely critical to timetable so precisely. Yes it has to share tracks with other services, yes time punctuality is more important now than historically but it isn't like having a lot f people on board all looking at their watch anxiously hoping to make a connection with another service- freight trains generally go from origin of journey of the goods direct to their destination, whereas passengers typically have a journey involving several trains and perhaps bus and if they just miss their connection by seconds they may have a long wait.
2. Braking is not so good and acceleration far poorer because of power to weight ratio. Also uses considerably more energy to speed up and a lot of brake wear and heat dissipation to slow down (whereas passenger stock often now uses braking that does not involve only friction against brake pads and increasingly can regenerate power to put into the traction system of an electrified railway.
3. Braking also takes a significant time to fully apply and also to cancel as it uses air pipe pressure along the length of a long train rather than generally electrical control over a typically much shorter passenger train. Therefore changing between braking and accelerating and then braking again isn't something to be undertaken if it can be avoided.
This all means that drivers of freight typically brake early to reduce their speed significantly, then cease active braking but coast at a very slowly decreasing speed until they need to brake again if they actually need to stop. The last thing they want to do is stop unnecessarily as that wastes energy and indeed the time needed to restart, so it is to their benefit to take as long as they can when approaching a signal at red to give it the best chance of clearing whilst they are still far enough away that they are still moving at a significant speed when it clears. Therefore over-braking is less of a concern for them than passenger train drivers for whom a few seconds of running time is more significant and therefore more prone to deciding to avoid braking sooner than they feel is really necessary.
This is a question that often catches people out and is worse than you are likely to encounter in the exam, but useful to find who is really understanding and who just puts numbers into formulae. Seems to me that you are in the first category.
(Tuesday, 3rd July, 2012 05:16)savitha kandasamy Wrote:
(Monday, 25th June, 2012 12:43)PJW Wrote: Generally seem to have made a good attempt.
You correctly determined that it is the passenger train rather than the freight train which is the more onerous constraint regarding the minimum signal spacing for braking, but it would have been good to have stated this explicitly.
Conversely you do not seem to have considered whether the capacity requirement needed for the freight trains at night gives rise to a more onerous constraint on the maximum spacing than that imposed by the passenger train required headway, or indeed the maximum amount of over-braking considered tolerable.
Also in the exam don't just quote the formula re Headway, but explain using a diagram its derivation.
As per the operational requirements for passenger trains,
Minimum Signal spacing = 0.5*BD1 = 0.5*2025 =1013m
Maximum Signal spacing, d =((Ht4*Vh)-(S+O+L))/3
= ((120*36)-640)/3
=1227m
As per the operational requirements for freight trains,
Minimum Signal spacing = 0.5*BD2 = 0.5*1458 =729m
Maximum Signal spacing, d =((Ht4*Vf)-(S+O+L))/3
= ((180*27)-850)/3
=1337m
The longest braking distance needs to be considered to ensure safety. So the minimum signal spacing is 1013m.
For Maximum signal spacing, if we considered freight train headway requirements as more onerous , we can place the signals at 1337m. This is within 33% of BD. But the headway requirements for passenger trains cannot be achieved with this maximum signal spacing. Also I am not sure how the braking distance requirement for freight will fit with the maximum signal spacing (if we choose either 1227m or 1337m). Kindly suggest me how to justify for the operational requirements for freight trains.
savitha
PJW