1998 calculation and layout

[greensky52]

Sorry, need much time to comprehend your meaning for my mother tongue is not Eng. Bad Eng for me....

But still can not understand all very well. Sometimes I am not sure I have got your points. So can we discuss on every topic gradually?

Firstly, 59s is for a train travelling from 1st cautionary aspect to stop signal. So I think in this period, the train should decellerate from 40kph to 0 mph, but not travel on an average speed, so only 325m(40kph/2*59s=325m) would be covered in this time. But referring to the study pack, it takes it as an average speed process.

Secondly, according to my understanding, you mean signal at rear of the platform 2200m--it must show a stop aspect when a train stopping in platform, and there must be another one at 1900m. There is 300m between these 2 signals, which is much more than the braking distance for 40kph(the braking distance for 40kph is 125m), it is long enough for the driver to stop the train at 2200m. So the signal at 1900m can be a cautionary aspect showing Yellow and 3 aspect can satisfy.

If choosing 4 aspect, the signal at 1900m will show a YY aspect, then it hints that the train will start brake at a Y aspect signal 300m prior to the YY one at 1900m. If so, the train will brake for 600m, which is much more longer than 123m, wasting! So 4 aspect is inappropriate.

Similiarly, 2 aspect is inappropriate too. Because it needs to put G/R signal at 2200m, another G/R one at 650m prior to 2200m, and also have to put a G/Y signal at 2075m(2200m-125m=2075m). It looks unproper for the station layout.

Is my comprehension the same with your meaning? I am not sure...
[/quote]

[PJW]

Sorry, need much time to comprehend your meaning for my mother tongue is not Eng. Bad Eng for me....

But still can not understand all very well. Sometimes I am not sure I have got your points. So can we discuss on every topic gradually?

Take your time. I'll try to express things clearly but this is difficult when they are complicated.
I don't know whether it is clearer to be vry brief or clearer to explain more fully.....

Firstly, 59s is for a train travelling from 1st cautionary aspect to stop signal. So I think in this period, the train should decelerate from 40kph to 0 mph, but not travel on an average speed, so only 325m(40kph/2*59s=325m) would be covered in this time. But referring to the study pack, it takes it as an average speed process.

The "59s" is the maximum running time that will allow the headway to be achieved and therefore a measure of how close the signals must be. Yes the train will need to decelerate from 11m/s which will take 22s and cover 125m. If we assume that from the slow speed that driver will indeed brake at the very last minute on sight of the red, then this leaves us 37s to cover at constant 11m/s so say 410m and therefore the maximum spacing would be 535m absolute maximum.
The Study Pack was not attempting actually to calculate this figure but only obtain an "order of magnitude" in order to argue that a mid platform signal was needed. Therefore it used a simplification of constant speed to say that the spacing could not be as high as 650m rather than waste time in getting a more accurate figure. Perhaps it would have been more clear if it read: Even at a constant 40kph, only 650m would be covered in 59s

The above calculation is probably appropriate for such a slow speed where the braking distance is less than sighting distance.
However your methodology is appropriate for the more usual case where the braking distance is significant but is actually rather less than the signal spacing. In reality the driver would significantly brake initially at the cautionary aspect, then coast (release the brakes but gradually slow because no power applied) and then brake again to a stand. This is difficult to calculate so a good approximation is to assume constant braking over whole length. Hence the time taken to cover the distance is indeed calculated by:
0.5 x (initial speed) x (distance).
I'd have used this methodolgy if initial speed had been 80km/h but not when the driver knows that the braking distance needed is so low compared to the sighting distance.

Secondly, according to my understanding, you mean signal at rear of the platform 2200m--it must show a stop aspect when a train stopping in platform, and there must be another one at 1900m. There is 300m between these 2 signals, which is much more than the braking distance for 40kph (the braking distance for 40kph is 125m), it is long enough for the driver to stop the train at 2200m. So the signal at 1900m can be a cautionary aspect showing Yellow and 3 aspect can satisfy.

If choosing 4 aspect, the signal at 1900m will show a YY aspect, then it hints that the train will start brake at a Y aspect signal 300m prior to the YY one at 1900m. If so, the train will brake for 600m, which is much more longer than 123m, wasting! So 4 aspect is inappropriate.

Similiarly, 2 aspect is inappropriate too. Because it needs to put G/R signal at 2200m, another G/R one at 650m prior to 2200m, and also have to put a G/Y signal at 2075m(2200m-125m=2075m). It looks unproper for the station layout.

Is my comprehension the same with your meaning? I am not sure...

I think so, but not 100% sure so will try to summarise again differently.

If signal 3 is at more than braking distance from signal 5, then a yellow aspect at signal 3 is sufficient warning for a red at signal 5 and therefore signal 1 can show Green. Good solution.

Keeping these signals in the same positions but implementing a 4 aspect sequence would make signal 1 show Double Yellow. this would be overbraked and also lower capacity as well as being more expensive; therefore this option would be stupid.

Putting in extra signals between signals 1&3 and between 3&5 and making them all 4 aspects is even more expensive but does give more capacity; it also keeps the "first warning" of the red in the same place as it was in the 3 aspect case. If this extra capacity is actually needed then it would be sensible, but in this case it is of no value as the inital 3 aspects are already suffficient. Hence to provide them would be waste of money and indeed having signals so close would give through reading problems; therefore this option would also be stupid.

In general where the spacing of stop signals needed to satisfy headway is much greater than braking distance, then 2 aspect Y/G and R/G pairs would be sensible; it avoids overbraking and de-couples the two considerations of headway and braking. This option would provide two pairs of 2 aspect signals rather than two 3 aspect signals and hence be more expensive and indeed in this specific case we would again have problems of "through reading" since braking distance is so short. Indeed overbraking is not a concern when signals so closely spaced anyway; hence this option would be stupid in this case.

Therefore 3 aspect signals are the sensible option here.

[greensky52]

Yeah, your state is very very clear to me this time. I am glad that through long and difficult discussion, I make clear of this with your help.

Sorry, need much time to comprehend your meaning for my mother tongue is not Eng. Bad Eng for me....

But still can not understand all very well. Sometimes I am not sure I have got your points. So can we discuss on every topic gradually?

Take your time. I'll try to express things clearly but this is difficult when they are complicated.
I don't know whether it is clearer to be vry brief or clearer to explain more fully.....

Firstly, 59s is for a train travelling from 1st cautionary aspect to stop signal. So I think in this period, the train should decelerate from 40kph to 0 mph, but not travel on an average speed, so only 325m(40kph/2*59s=325m) would be covered in this time. But referring to the study pack, it takes it as an average speed process.

The "59s" is the maximum running time that will allow the headway to be achieved and therefore a measure of how close the signals must be. Yes the train will need to decelerate from 11m/s which will take 22s and cover 125m. If we assume that from the slow speed that driver will indeed brake at the very last minute on sight of the red, then this leaves us 37s to cover at constant 11m/s so say 410m and therefore the maximum spacing would be 535m absolute maximum.
The Study Pack was not attempting actually to calculate this figure but only obtain an "order of magnitude" in order to argue that a mid platform signal was needed. Therefore it used a simplification of constant speed to say that the spacing could not be as high as 650m rather than waste time in getting a more accurate figure. Perhaps it would have been more clear if it read: Even at a constant 40kph, only 650m would be covered in 59s

The above calculation is probably appropriate for such a slow speed where the braking distance is less than sighting distance.
However your methodology is appropriate for the more usual case where the braking distance is significant but is actually rather less than the signal spacing. In reality the driver would significantly brake initially at the cautionary aspect, then coast (release the brakes but gradually slow because no power applied) and then brake again to a stand. This is difficult to calculate so a good approximation is to assume constant braking over whole length. Hence the time taken to cover the distance is indeed calculated by:
0.5 x (initial speed) x (distance).
I'd have used this methodolgy if initial speed had been 80km/h but not when the driver knows that the braking distance needed is so low compared to the sighting distance.

Secondly, according to my understanding, you mean signal at rear of the platform 2200m--it must show a stop aspect when a train stopping in platform, and there must be another one at 1900m. There is 300m between these 2 signals, which is much more than the braking distance for 40kph (the braking distance for 40kph is 125m), it is long enough for the driver to stop the train at 2200m. So the signal at 1900m can be a cautionary aspect showing Yellow and 3 aspect can satisfy.

If choosing 4 aspect, the signal at 1900m will show a YY aspect, then it hints that the train will start brake at a Y aspect signal 300m prior to the YY one at 1900m. If so, the train will brake for 600m, which is much more longer than 123m, wasting! So 4 aspect is inappropriate.

Similiarly, 2 aspect is inappropriate too. Because it needs to put G/R signal at 2200m, another G/R one at 650m prior to 2200m, and also have to put a G/Y signal at 2075m(2200m-125m=2075m). It looks unproper for the station layout.

Is my comprehension the same with your meaning? I am not sure...

I think so, but not 100% sure so will try to summarise again differently.

If signal 3 is at more than braking distance from signal 5, then a yellow aspect at signal 3 is sufficient warning for a red at signal 5 and therefore signal 1 can show Green. Good solution.

Keeping these signals in the same positions but implementing a 4 aspect sequence would make signal 1 show Double Yellow. this would be overbraked and also lower capacity as well as being more expensive; therefore this option would be stupid.

Putting in extra signals between signals 1&3 and between 3&5 and making them all 4 aspects is even more expensive but does give more capacity; it also keeps the "first warning" of the red in the same place as it was in the 3 aspect case. If this extra capacity is actually needed then it would be sensible, but in this case it is of no value as the inital 3 aspects are already suffficient. Hence to provide them would be waste of money and indeed having signals so close would give through reading problems; therefore this option would also be stupid.

In general where the spacing of stop signals needed to satisfy headway is much greater than braking distance, then 2 aspect Y/G and R/G pairs would be sensible; it avoids overbraking and de-couples the two considerations of headway and braking. This option would provide two pairs of 2 aspect signals rather than two 3 aspect signals and hence be more expensive and indeed in this specific case we would again have problems of "through reading" since braking distance is so short. Indeed overbraking is not a concern when signals so closely spaced anyway; hence this option would be stupid in this case.

Therefore 3 aspect signals are the sensible option here.