I look at the calcs later. In brief:
Layout-
Signals 11 and 13 now only read to one destination- therefore no route indicators.
Signal 16 has 2 routes, so just a pos 4 PLJI for the slower speed route.
Need GPL at the junction at station A so that loco can use the railing crossover. Also signal 27 would need a PL so that loco can reattach to its train (make sure that the overlap of 27 is not going to be occupied by the 400m train tailing back).
28 signal doesn't need a PL. It has only one route so do not need the PLJIs; however there is an argument for giving one PLJI (pos1) for the move across to the correct running line, though since the signal can't be approached I don't think I'd provide it.
31 signal wouldn't have PLJI since the move over the crossover is authorised by the PL not the main aspect.
Don't need signal 33.
1. The freight train is 400m long. The locomotive cannot run around purely at C. It has to leave its train behind at signal 31, itself cross to the LOS and travel to station A on one line and then back on the other in order to couple onto the other end of the wagons. Then somehow the whole train needs to go back to station A so that it then goes the other way at the junction.
2. Yes. In the UK a driver is trained to brake on seeing the yellow. If the next signal is very much more than braking distance then will brake until running slowly and then continue until sees the red. Of course if this becomes the norm then drivers will tend not to brake so hard when seeing a yellow. This can be dangerous if they get accustomed to doing this because when encounter a signal at yellow which is only just at braking, then they may not brake enough and then fail to stop in time when getting to the red signal. Therefore we generally attempt to avoid signals being spaced at more than 133 percent of braking. If headway requirements are low (i.e. few trains per hour), then we use 2 aspect signals, with the red/greens spaced dictating the headway, each having its own yellow/green at braking distance on the approach; it is 3 aspect signalling but no one signal shows more than 2 colours.
3. I'll comment on these later.
My description of stopping headway reflects modern "defensive driving" and if you look in older books the methodology used there would give a better headway (but relied on drivers not driving more cautiously when the signal prior to the station at which they are to stop anyway displays yellow compared to green). This may make quite a difference to the numbers.
Layout-
Signals 11 and 13 now only read to one destination- therefore no route indicators.
Signal 16 has 2 routes, so just a pos 4 PLJI for the slower speed route.
Need GPL at the junction at station A so that loco can use the railing crossover. Also signal 27 would need a PL so that loco can reattach to its train (make sure that the overlap of 27 is not going to be occupied by the 400m train tailing back).
28 signal doesn't need a PL. It has only one route so do not need the PLJIs; however there is an argument for giving one PLJI (pos1) for the move across to the correct running line, though since the signal can't be approached I don't think I'd provide it.
31 signal wouldn't have PLJI since the move over the crossover is authorised by the PL not the main aspect.
Don't need signal 33.
1. The freight train is 400m long. The locomotive cannot run around purely at C. It has to leave its train behind at signal 31, itself cross to the LOS and travel to station A on one line and then back on the other in order to couple onto the other end of the wagons. Then somehow the whole train needs to go back to station A so that it then goes the other way at the junction.
2. Yes. In the UK a driver is trained to brake on seeing the yellow. If the next signal is very much more than braking distance then will brake until running slowly and then continue until sees the red. Of course if this becomes the norm then drivers will tend not to brake so hard when seeing a yellow. This can be dangerous if they get accustomed to doing this because when encounter a signal at yellow which is only just at braking, then they may not brake enough and then fail to stop in time when getting to the red signal. Therefore we generally attempt to avoid signals being spaced at more than 133 percent of braking. If headway requirements are low (i.e. few trains per hour), then we use 2 aspect signals, with the red/greens spaced dictating the headway, each having its own yellow/green at braking distance on the approach; it is 3 aspect signalling but no one signal shows more than 2 colours.
3. I'll comment on these later.
My description of stopping headway reflects modern "defensive driving" and if you look in older books the methodology used there would give a better headway (but relied on drivers not driving more cautiously when the signal prior to the station at which they are to stop anyway displays yellow compared to green). This may make quite a difference to the numbers.
(05-11-2011, 03:48 PM)onestrangeday Wrote: Hi PJW:
Thank you for taking time to review my attempt and your suggestions are very valuable for me for learning. I have attached my revised version for headway calculations of Q2-4 and the layout, please have a look to see whether I am on the track or not.
I would like to ask more questions if it is ok:
1. I think I need to understand the question clearly before I can put on signals on the layout especially to meet the operational requirement.
So my question is On the question it described “Freight - Line A to Line B reversing at Station C, running around via crossovers at Station A and Station C - 1 per night (0100-0600)”.
My interpretation is that the freight train first travel from line A and stop at station C. After that it reverses from the loop in station C and make a cross to the lower line using the crossover at station C and return back to Station A. When it arrives at station A, the freight train makes another cross to the upper line using the crossover at station A.
Or does it depends on how you comprehend the question, I mean the description ?? or may be I pay too much concentration on this ?
2. You have mentioned the following
“You should also have considered the maximum spacing constraint dictated by the degree of excess braking considered acceptable (as above) in case it is more onerous than that given by headway consideration; in this case it is not the constraint but a statement to that effect should have been included.”
Do you mean that when design the signalling layout, in real situation the maximum spacing will be shorter than the theoretical value because of excess braking taken as a major factor ?
3. The result I’ve calculated for Question 4 is differ to what you have presumed (the distance is greater than 3500m), can you check my result ?
Thanks
PJW