Re the 2005 layout- I am looking at the one posted by Alex.
The reason for the 4 aspects is because there is a need for signals to protect junctions etc that are spaced closer together than braking distance.
Simply has to be a signal to protect junction D- hence 129.
Similarly must be signal to protect the turnback siding, hence 121.
Definitely needs to be a signal to protect the station throat- I'd probably have wanted to place 117/119 at least a little further back so that their overlaps wouldn't conflict with those of the other direction platform starters, but broadly do need to be around here.
In the opposite direction
it is clear that 128 is in a sensible position and that 120 simply has to go more or less exactly where it is. 1
24 isn't absolutely essential perhaps, but given that there does need to be a signal on the Up Slow somewhere around 126 (actually I would want to put it rather closer to the pointwork and maximise the standage for a train waiting at it clear of junction D) then certainly 124 is sensible.
Do remember as well that when the speed is nominally high through a station where trains stop, then those trains are travelling at a small percentage of full speed as they approach and depart, so having those block sections short is very beneficial as it keeps the time taken to traverse them more compatible with the longer sections far away that are covered at full speed and capacity is limited by the longest (in terms of time) section.
Hence considerations of protecting the layout junctions means that we will want signals in such positions and these seem to average around 700m apart in this vicinit; since this is less than the calculated braking distance there are only 3 options-
1. 4 aspect signalling for this portion of line (even though not needed when solely thinking about capacity for headway);
2. Make 3 aspect signal approach released if the following signal is at red- only really an option for a single underbraked signal section and has the disadvantage of crippling capacity;
3. Reduce the speed of the line through the affected area- you perhaps could have argued this as a solution if all trains stopped at C (but of course you'd have needed to ensure that the desired capacity could still be achieved)
Therefore just on the basis of looking at the layout without worrying about the calcs, the decision to have 4 aspects in the central area is looking eminently sensible. Less convinced that there should be 4 aspects at 133, 134. Conversely the transition from 3 aspect from the left seems sensible- note how it is the second of the "4 aspects" which has no red- since there isn't a signal braking distance in rear of it. My gut feeling is that 130 signal should have been provided as a non-red signal and then that end would have worked sensibly as well; could even be forgiven that the layout had actually been designed by the examiner with signals in such positions.......
So on the face of it the calculations were corect, the need (despite these) is for there to be some 4 aspect signals; their mistake was not to add onto the end of their calculations an EXPLANATION.
Actually if you look at the Study Pack you'll see I recommend starting the paper by looking at all the points, put the relevant track jointa in and pencilling some possibilities for signals around the key junction areas and thereby get a feel for the whole layout BEFORE sitting down to churn the numbers in the calcs- this means you can immediately INTERPRET them rather than follow slavishly.
Don't forget that the examiners will not be wanting you to put loads of the same form of sigalling on a layout; if there isn't a long lightly used branch, a rolling-stock depot, a freight terminal etc., then "a pound to a penny" that there will be a need for at least one aspect sequence transition in what otherwise looks to be homogeneous railway.
You would not have suffered purely BECAUSE you didn't put in 4 aspects,
BUT unless you had signalled the layout in a way which was
1. safe,
2. met the capacity and other specified operational requirements, and
3. as efficient and economical manner as the other constraints permit,
then YES your marks would have suffered.
You need to recognise that MAS signals are placed in positions detemined by
a) where it is sensible to have a red aspect [protecting conflicts, giving standage etc], and
b) where it is sensible to provide the caution aspect(s) [enough braking distance to the red but not very excessive distance]
c) where there is need for a green to be given for the 2nd train not to have to start to brake because of the presence of the traib ahead [satisy the headway requirement]
Your solution needs to balance all these constraints and you need to know what are good practice constraints which may occasionally have to be enfringed for the greater good of the whole when compromises need to be made, and which of them must simply always be satisfied.
There's no "quick win" but if you plough through the study pack and look at the examples and comments on this website (as you clearly have) then you'll actually pick up the information in a concentrated form that would have taken very many years of design experience to acquire. At least one Distinction has been gained by someone who has never been employed in a signalling design office, so it is perfectly possible, but I didn't say it was easy.
The reason for the 4 aspects is because there is a need for signals to protect junctions etc that are spaced closer together than braking distance.
Simply has to be a signal to protect junction D- hence 129.
Similarly must be signal to protect the turnback siding, hence 121.
Definitely needs to be a signal to protect the station throat- I'd probably have wanted to place 117/119 at least a little further back so that their overlaps wouldn't conflict with those of the other direction platform starters, but broadly do need to be around here.
In the opposite direction
it is clear that 128 is in a sensible position and that 120 simply has to go more or less exactly where it is. 1
24 isn't absolutely essential perhaps, but given that there does need to be a signal on the Up Slow somewhere around 126 (actually I would want to put it rather closer to the pointwork and maximise the standage for a train waiting at it clear of junction D) then certainly 124 is sensible.
Do remember as well that when the speed is nominally high through a station where trains stop, then those trains are travelling at a small percentage of full speed as they approach and depart, so having those block sections short is very beneficial as it keeps the time taken to traverse them more compatible with the longer sections far away that are covered at full speed and capacity is limited by the longest (in terms of time) section.
Hence considerations of protecting the layout junctions means that we will want signals in such positions and these seem to average around 700m apart in this vicinit; since this is less than the calculated braking distance there are only 3 options-
1. 4 aspect signalling for this portion of line (even though not needed when solely thinking about capacity for headway);
2. Make 3 aspect signal approach released if the following signal is at red- only really an option for a single underbraked signal section and has the disadvantage of crippling capacity;
3. Reduce the speed of the line through the affected area- you perhaps could have argued this as a solution if all trains stopped at C (but of course you'd have needed to ensure that the desired capacity could still be achieved)
Therefore just on the basis of looking at the layout without worrying about the calcs, the decision to have 4 aspects in the central area is looking eminently sensible. Less convinced that there should be 4 aspects at 133, 134. Conversely the transition from 3 aspect from the left seems sensible- note how it is the second of the "4 aspects" which has no red- since there isn't a signal braking distance in rear of it. My gut feeling is that 130 signal should have been provided as a non-red signal and then that end would have worked sensibly as well; could even be forgiven that the layout had actually been designed by the examiner with signals in such positions.......
So on the face of it the calculations were corect, the need (despite these) is for there to be some 4 aspect signals; their mistake was not to add onto the end of their calculations an EXPLANATION.
Actually if you look at the Study Pack you'll see I recommend starting the paper by looking at all the points, put the relevant track jointa in and pencilling some possibilities for signals around the key junction areas and thereby get a feel for the whole layout BEFORE sitting down to churn the numbers in the calcs- this means you can immediately INTERPRET them rather than follow slavishly.
Don't forget that the examiners will not be wanting you to put loads of the same form of sigalling on a layout; if there isn't a long lightly used branch, a rolling-stock depot, a freight terminal etc., then "a pound to a penny" that there will be a need for at least one aspect sequence transition in what otherwise looks to be homogeneous railway.
You would not have suffered purely BECAUSE you didn't put in 4 aspects,
BUT unless you had signalled the layout in a way which was
1. safe,
2. met the capacity and other specified operational requirements, and
3. as efficient and economical manner as the other constraints permit,
then YES your marks would have suffered.
You need to recognise that MAS signals are placed in positions detemined by
a) where it is sensible to have a red aspect [protecting conflicts, giving standage etc], and
b) where it is sensible to provide the caution aspect(s) [enough braking distance to the red but not very excessive distance]
c) where there is need for a green to be given for the 2nd train not to have to start to brake because of the presence of the traib ahead [satisy the headway requirement]
Your solution needs to balance all these constraints and you need to know what are good practice constraints which may occasionally have to be enfringed for the greater good of the whole when compromises need to be made, and which of them must simply always be satisfied.
There's no "quick win" but if you plough through the study pack and look at the examples and comments on this website (as you clearly have) then you'll actually pick up the information in a concentrated form that would have taken very many years of design experience to acquire. At least one Distinction has been gained by someone who has never been employed in a signalling design office, so it is perfectly possible, but I didn't say it was easy.
Anonymous Wrote:I am studying for Mod 2- to date it is going well, although I am finding the stopping headway difficult to interpret. I have started applying the logic to the layout and it is coming along slowly, I have one query which leads to a number of questions concerning the 2005 layout produced oby two candidates, both have worked out the stopping and non-stopping headway and recommend 3 aspect signalling, yet when they have signalled the layout they have installed four aspect signalling around the main station area.
How have they arrived at this conclusion?
What is their calculated distances for signal spacing?
In my case I would have struggled to interpret this- would I suffer from this decision?
Is there any quick calculation or guidance that could lead me to a similar decision, given my lack of design experience?
(01-10-2010, 10:33 AM)greensky52 Wrote: Thanks, friends. Suggestions in last time are all useful. Thanks again.
I have been reading Mod 5 and reviewing Mod 3 as a relexing these two days.
Anyway, result is not the most important, More important is I learnt much here.
PJW