another try at double line layout

[onestrangeday]

Sure, it's great help PJW.
I'll wait for your comment for my calculation.


thanks

I look at the calcs later. In brief:

[PJW]

In general they seem fine until you lose me a bit on the last page.The easier way to calculate the time taken to decelerate at constant bake rate to come to rest (especially where that rate is 0.5m/s/s) is to take the inital speed and divide by the rate- train initally at 33.3m/s and in every second it loses 0.5m/s then clearly it will take 66.6s to stop.

Para re acceleration being symmetrical with braking is true, but not sure what use you are making of that fact.

The times you calculate between train claering the overlap and covering its own length are based on 33.3m/s. Looks to me as if when you say "the signal" you were thinking of the signal one section on from the station. However the pinch point as far as capactity is concerned is for the section which includes the station- afterall for 30 seconds the train is not moving at all and it also is moving slowly as it arrives and departs. All this time the signal protecting the station is held to red and it is this (and the consequential restrictive aspect at the outer signal) that is giving the capacity constraint.

So although you have got the individual elements,you are not quite calculating what you need to do. The calculation for the train to move its own length plus the length of the overlap is from the standing start at the station.

Let's consider station B.
Until the rear of the earlier train has cleared the overlap beyond signal 23, signal 19 is held to red and therefore the outer signal (which if we work on the basis of you disgram as it currently exists) is signal 15 that can only be at yellow. Therefore any driver seeing this signal would not accelerate up to their usual speed as they would know that the next signal could be at red and therefore they would have to limit their speed so that they could stop within the distance for which it is visible, or else they risk SPADing it.
Much the same would be true if there were an intermediate signal 17 in the length between the stations. What we are trying to calculate is how far away from station B we could place this signal and yet still get it to show a green by the time that a driver approaching at their normal running speed first sees it, so that we avoid them needing to brake.

Then we use the method of calculation you have deployed to add the time that this 2nd train the needs to get into the station and dwell so that it is itself ready to leave, with the time the first train took to accelerate so that its rear end had cleared the overlap beyond signal 23.

There are some similarities between this layout and a previous IRSE Exam layout; have a look at this thread re 2000 Headway Calcs and layout
In particular I have placed a couple of attachments with graphs of stopping headway for 3 aspect and also a comparison between providing 3 aspect and 4 aspect signalling.

Sure, it's great help PJW.
I'll wait for your comment for my calculation.


thanks

I look at the calcs later. In brief:

[onestrangeday]

Hi PJW:

many thanks for you have taken time to review my attempt and I have understood the subject much better.

For Question 2:
You have mentioned of
"the times you calculate between train claering the overlap and covering its own length are based on 33.3m/s. Looks to me as if when you say "the signal" you were thinking of the signal one section on from the station. However the pinch point as far as capactity is concerned is for the section which includes the station- afterall for 30 seconds the train is not moving at all and it also is moving slowly as it arrives and departs. All this time the signal protecting the station is held to red and it is this (and the consequential restrictive aspect at the outer signal) that is giving the capacity constraint."

I calculated the stopping headway for passenger trains using the "Signal 2" in my calculation to prove that the 3-aspects signalling system accommodate both stopping and non-stopping headway requirements . For 'signal 2' I mean the diagram found in the "Railway Signaling" textbook on page 23. Or do you think I should use "the green headway on signal 1" as for the proof ?

For Question 4:

BUT for Question four, I use the "Signal 1" for stopping headway calculation and further derived the signal spacing that could be allowed in station C.

However, as described in the "Railway Signaling" textbook on page 23, it also mentions that

"The Green headway on signal 1 governs the service, because, although the Green headway on Signal 2 is greater, trains will enter the station on a Yellow aspect."

So I think this is what you have mentioned about in your explanation that I should use the 'Green headway on Signal 1' for determining the signal spacing for stopping trains at station C. Am I right ?

sorry to keep asking questions.



thanks

In general they seem fine until you lose me a bit on the last page.The easier way to calculate the time taken to decelerate at constant bake rate to come to rest (especially where that rate is 0.5m/s/s) is to take the inital speed and divide by the rate- train initally at 33.3m/s and in every second it loses 0.5m/s then clearly it will take 66.6s to stop.

Para re acceleration being symmetrical with braking is true, but not sure what use you are making of that fact.

The times you calculate between train claering the overlap and covering its own length are based on 33.3m/s. Looks to me as if when you say "the signal" you were thinking of the signal one section on from the station. However the pinch point as far as capactity is concerned is for the section which includes the station- afterall for 30 seconds the train is not moving at all and it also is moving slowly as it arrives and departs. All this time the signal protecting the station is held to red and it is this (and the consequential restrictive aspect at the outer signal) that is giving the capacity constraint.

So although you have got the individual elements,you are not quite calculating what you need to do. The calculation for the train to move its own length plus the length of the overlap is from the standing start at the station.

Let's consider station B.
Until the rear of the earlier train has cleared the overlap beyond signal 23, signal 19 is held to red and therefore the outer signal (which if we work on the basis of you disgram as it currently exists) is signal 15 that can only be at yellow. Therefore any driver seeing this signal would not accelerate up to their usual speed as they would know that the next signal could be at red and therefore they would have to limit their speed so that they could stop within the distance for which it is visible, or else they risk SPADing it.
Much the same would be true if there were an intermediate signal 17 in the length between the stations. What we are trying to calculate is how far away from station B we could place this signal and yet still get it to show a green by the time that a driver approaching at their normal running speed first sees it, so that we avoid them needing to brake.

Then we use the method of calculation you have deployed to add the time that this 2nd train the needs to get into the station and dwell so that it is itself ready to leave, with the time the first train took to accelerate so that its rear end had cleared the overlap beyond signal 23.

There are some similarities between this layout and a previous IRSE Exam layout; have a look at this thread re 2000 Headway Calcs and layout
In particular I have placed a couple of attachments with graphs of stopping headway for 3 aspect and also a comparison between providing 3 aspect and 4 aspect signalling.

Sure, it's great help PJW.
I'll wait for your comment for my calculation.


thanks

I look at the calcs later. In brief:

[PJW]

Asking questions is the way to learn.

The capacity of a line is limited by the worst section.
In the real world, where all sections will probably be of slightly different length,gradient and speed profile then each section must be checked. Generally in the exam a nominal spacing is assumed and the railway is homogeneous, so for non-stop headway calculation for any one section is sufficient. Otherwise go for the obviously worst one (e.g. if other things are equal, then the longest).

For the stopping headway, the fact that the train stops at the station makes this (if of the same physical length as the others) the longest section in terms of time. Therefore it is the effect of this section which almost certainly dominates.

Trains are at minimum headway when the driving of train 2 is not affected by the presence of train 1. In the non-stop case it is obvious that this means just not seeing an aspect which is less than green. In the case of stopping it isn't quite so clear.

A) Imagine the limiting case of signals spaced at minimum braking and the headway speed equal to the maximum permissible speed. Whatever the aspect of "signal 2", the driver will start braking thereso as to stop their train in the platform (and thus incidentally at "signal 3"). In that case it certainly doesn't matter whether the driver passes "signal 2" at yellow or green; their driving would be the same.

B) However if the train were travelling more slowly as it would if the headway speed were less than the maximum speed, the driver wouldn't actually need to start braking at "signal 2" and could leave it a bit later to brake. The same would be true even if travelling at maximum permissible speed if the driver knew that the signals were actually spaced at a bit more than braking distance. In the 1980s it would have been regarded as the driver's perogative to decide where to start braking; indeed they were indeed encouraged to leave as late as possible in an attempt to keep time and to maximise the capacity of the line. Hence a yellow aspect on "signal 2" would have been viewed as irrelevant for a train which was going to be stopping at the station regardless. Of course a driver would be a bit more cautious when getting a yellow; if they ended up overshooting the platform by a few metres it probably wouldn't matter and was unlikely to be found out by those in authority anyway. In the worst case that significantly overshot then could choose not to stop- annoyed passengers on board and waiting to board but no chance of an accident.
Conversely if a signal were SPADed by a few metres it would be enough to drop the track circuit beyond and this would mean that it was unlikely to be covered up and there would be an inquiry and a blot on their record; obviously if the overrun were long then there would be a safety risk.

C) Nowadays things have changed. Society is far more risk adverse and also litigious. SPADs are more high profile. Drivers more likely to be removed from their job even for relatively minor incidents. Much more monitoring, aided by data recorders both on-board the train and at the signalling centre. Drivers now trained that they must make a significant brake application when sighting a restrictive aspect, even if already travelling relatively slowly.
If the On Board Data Recorder showed a driver continuing to run at headway speed having passed a yellow as depicted in Fig 2:9 then they would be up for an interview with their Train Crew Manager. Therefore there is today a definite difference to the driving of the second train if "signal2" is at yellow.

Hence this is why I recommended the approach described earlier. Yes it is at variance to the Railway Signalling text book, but that book is now over 30 years old and things have changed since- a not inconsiderable period in the history of signalling. For comparison, see the simplistic summary below, to place in context:
1830 - 1860: Very embryonic if anyl
1860 - 1890: Gradual introduction of what is now recognisable (and in some cases still in use!)
1890 - 1920: Mechanical signalling largely perfected, experimentation with power signalling
1920 - 1950: Multiple aspect signalling becomes established standard
1950 - 1980: Large panel boxes with route relay interlocking
1980 - 2010: VDU control centres with electronic interlockings, increased automation in route setting, train protection etc.
2010 - 2040: Gradual elimination of lineside signalling as migrates to be transmission based in-cab.....

For a real job tend to use more sophisticated computer simulations using precise proposed signal positions and sample possible timetables t demnstrate capacity rather than such methods. The important thing for the exam is to show you understand the concept and don't regard the computer's output as "magic". If you were really doing longhand, then you'd actually have to do for every section for the line. You'll notice in the textbook the Green headways at "signal 1" and "signal 2" are given as 281 and 285sec- hardly a big difference and it would not take much inequality in signal spacing to affect this. So if you are more comfortable following the text book that is fine, but I was trying to show what would be more realistic in curent environment.

Hi PJW:

many thanks for you have taken time to review my attempt and I have understood the subject much better.

For Question 2:
You have mentioned of
"the times you calculate between train claering the overlap and covering its own length are based on 33.3m/s. Looks to me as if when you say "the signal" you were thinking of the signal one section on from the station. However the pinch point as far as capactity is concerned is for the section which includes the station- afterall for 30 seconds the train is not moving at all and it also is moving slowly as it arrives and departs. All this time the signal protecting the station is held to red and it is this (and the consequential restrictive aspect at the outer signal) that is giving the capacity constraint."

I calculated the stopping headway for passenger trains using the "Signal 2" in my calculation to prove that the 3-aspects signalling system accommodate both stopping and non-stopping headway requirements . For 'signal 2' I mean the diagram found in the "Railway Signaling" textbook on page 23. Or do you think I should use "the green headway on signal 1" as for the proof ?

For Question 4:

BUT for Question four, I use the "Signal 1" for stopping headway calculation and further derived the signal spacing that could be allowed in station C.

However, as described in the "Railway Signaling" textbook on page 23, it also mentions that

"The Green headway on signal 1 governs the service, because, although the Green headway on Signal 2 is greater, trains will enter the station on a Yellow aspect."

So I think this is what you have mentioned about in your explanation that I should use the 'Green headway on Signal 1' for determining the signal spacing for stopping trains at station C. Am I right ?

sorry to keep asking questions.

thanks

[onestrangeday]

Hi PJW:

Thanks you for your detailed explanation, it certainly gives me more insight into the subject and more importantly understand the concept. I agree with you that nowadays the signal spacing are calculated by computer simulation which can incorporated all the relevant factors into consideration and certainly will make less errors and more efficient as compare with the work done by human. Personally, I am more interested to know about the realistic situation rather than for examination purpose. I think the examination is just a way or evidence to prove that you could do ‘something’ but not saying you are able ‘do’ a thing. Anyway do you think the IRSE shall take some efforts to update its textbook as to cope with the fast changing world? But I guess this will take a long time and much contribution from many professionals in the industry to complete the task.


Thanks your explanation, I believe I will ask more questions in the future

Asking questions is the way to learn.

The capacity of a line is limited by the worst section.
In the real world, where all sections will probably be of slightly different length,gradient and speed profile then each section must be checked. Generally in the exam a nominal spacing is assumed and the railway is homogeneous, so for non-stop headway calculation for any one section is sufficient. Otherwise go for the obviously worst one (e.g. if other things are equal, then the longest).

For the stopping headway, the fact that the train stops at the station makes this (if of the same physical length as the others) the longest section in terms of time. Therefore it is the effect of this section which almost certainly dominates.

Trains are at minimum headway when the driving of train 2 is not affected by the presence of train 1. In the non-stop case it is obvious that this means just not seeing an aspect which is less than green. In the case of stopping it isn't quite so clear.

A) Imagine the limiting case of signals spaced at minimum braking and the headway speed equal to the maximum permissible speed. Whatever the aspect of "signal 2", the driver will start braking thereso as to stop their train in the platform (and thus incidentally at "signal 3"). In that case it certainly doesn't matter whether the driver passes "signal 2" at yellow or green; their driving would be the same.

B) However if the train were travelling more slowly as it would if the headway speed were less than the maximum speed, the driver wouldn't actually need to start braking at "signal 2" and could leave it a bit later to brake. The same would be true even if travelling at maximum permissible speed if the driver knew that the signals were actually spaced at a bit more than braking distance. In the 1980s it would have been regarded as the driver's perogative to decide where to start braking; indeed they were indeed encouraged to leave as late as possible in an attempt to keep time and to maximise the capacity of the line. Hence a yellow aspect on "signal 2" would have been viewed as irrelevant for a train which was going to be stopping at the station regardless. Of course a driver would be a bit more cautious when getting a yellow; if they ended up overshooting the platform by a few metres it probably wouldn't matter and was unlikely to be found out by those in authority anyway. In the worst case that significantly overshot then could choose not to stop- annoyed passengers on board and waiting to board but no chance of an accident.
Conversely if a signal were SPADed by a few metres it would be enough to drop the track circuit beyond and this would mean that it was unlikely to be covered up and there would be an inquiry and a blot on their record; obviously if the overrun were long then there would be a safety risk.

C) Nowadays things have changed. Society is far more risk adverse and also litigious. SPADs are more high profile. Drivers more likely to be removed from their job even for relatively minor incidents. Much more monitoring, aided by data recorders both on-board the train and at the signalling centre. Drivers now trained that they must make a significant brake application when sighting a restrictive aspect, even if already travelling relatively slowly.
If the On Board Data Recorder showed a driver continuing to run at headway speed having passed a yellow as depicted in Fig 2:9 then they would be up for an interview with their Train Crew Manager. Therefore there is today a definite difference to the driving of the second train if "signal2" is at yellow.

Hence this is why I recommended the approach described earlier. Yes it is at variance to the Railway Signalling text book, but that book is now over 30 years old and things have changed since- a not inconsiderable period in the history of signalling. For comparison, see the simplistic summary below, to place in context:
1830 - 1860: Very embryonic if anyl
1860 - 1890: Gradual introduction of what is now recognisable (and in some cases still in use!)
1890 - 1920: Mechanical signalling largely perfected, experimentation with power signalling
1920 - 1950: Multiple aspect signalling becomes established standard
1950 - 1980: Large panel boxes with route relay interlocking
1980 - 2010: VDU control centres with electronic interlockings, increased automation in route setting, train protection etc.
2010 - 2040: Gradual elimination of lineside signalling as migrates to be transmission based in-cab.....

For a real job tend to use more sophisticated computer simulations using precise proposed signal positions and sample possible timetables t demnstrate capacity rather than such methods. The important thing for the exam is to show you understand the concept and don't regard the computer's output as "magic". If you were really doing longhand, then you'd actually have to do for every section for the line. You'll notice in the textbook the Green headways at "signal 1" and "signal 2" are given as 281 and 285sec- hardly a big difference and it would not take much inequality in signal spacing to affect this. So if you are more comfortable following the text book that is fine, but I was trying to show what would be more realistic in curent environment.

Hi PJW:

many thanks for you have taken time to review my attempt and I have understood the subject much better.

For Question 2:
You have mentioned of
"the times you calculate between train claering the overlap and covering its own length are based on 33.3m/s. Looks to me as if when you say "the signal" you were thinking of the signal one section on from the station. However the pinch point as far as capactity is concerned is for the section which includes the station- afterall for 30 seconds the train is not moving at all and it also is moving slowly as it arrives and departs. All this time the signal protecting the station is held to red and it is this (and the consequential restrictive aspect at the outer signal) that is giving the capacity constraint."

I calculated the stopping headway for passenger trains using the "Signal 2" in my calculation to prove that the 3-aspects signalling system accommodate both stopping and non-stopping headway requirements . For 'signal 2' I mean the diagram found in the "Railway Signaling" textbook on page 23. Or do you think I should use "the green headway on signal 1" as for the proof ?

For Question 4:

BUT for Question four, I use the "Signal 1" for stopping headway calculation and further derived the signal spacing that could be allowed in station C.

However, as described in the "Railway Signaling" textbook on page 23, it also mentions that

"The Green headway on signal 1 governs the service, because, although the Green headway on Signal 2 is greater, trains will enter the station on a Yellow aspect."

So I think this is what you have mentioned about in your explanation that I should use the 'Green headway on Signal 1' for determining the signal spacing for stopping trains at station C. Am I right ?

sorry to keep asking questions.

thanks

[PJW]

I think you are viewing the examination in exactly tthe right way.

I am not sure that updating that textbook is the answer; technology is evolving faster than it was and the variety of railways which IRSE memeners need to cope with is far wider than ever. But yes, I do think that something better to help people studying for the Exam is required. The Study Packs could and should be updated.
Actually I have submitted an abstract of a possible paper to the "Aspect 2012"committee for cosideration. Let's see if they want me to present at the Conference.......

Personally, I am more interested to know about the realistic situation rather than for examination purpose. I think the examination is just a way or evidence to prove that you could do ‘something’ but not saying you are able ‘do’ a thing.

Anyway do you think the IRSE shall take some efforts to update its textbook as to cope with the fast changing world? But I guess this will take a long time and much contribution from many professionals in the industry to complete the task.
.