2000 Headway Calcs and Layout

[alexgoei]

Has that made it clearer or confused you now completely?

Hello PJW,

Thanks for clarifying when to use (a) clearing the overlap in order for signals to clear for the next following train as part of identifying pinch points and (b) non stopping train following stopping train (not stopping following stopping).

The DGR from the Non Stopping Headway computation was used as the starting point for determining signal spacing for computing the stopping headway although this may subsequently change to meet the stopping headway requirements. Is this incorrect?

I have had a chance to compare the workings of two other students on this forum; one by "interesting signals" which is 3 aspect and the other by "douglas" which is 4 aspect. Both after some reworking and your comments give almost identical headways of about 244 secs. I am trying to determine which is operationally superior. Will let you know.

Regards

Alex

[PJW]

Has that made it clearer or confused you now completely?

The DGR from the Non Stopping Headway computation was used as the starting point for determining signal spacing for computing the stopping headway although this may subsequently change to meet the stopping headway requirements. Is this incorrect?

Certainly need to prove that THE SAME proposed signalling meets BOTH requirements. It may be that you can get a good idea from looking at the respective figures which is the likely to be the more onerous- typically (in questions set in IRSE Exam) a stopping train will lose approx 30s during braking, 30s dwell and approx 30s during acceleration. Hence is the Non-stop requirement is 2.5mins and Stopping is 3.5mins then it is obvious that the latter is dominant; if the Stopping however is 5 mins then it is unlikely to be the important factor.

I'd generally worry about the Non Stop requirement first- after all that would allows you to signal a good proportion of the layout reasonably. Note that you may be asked to calculate the best possible headway that could be achieved, but when placing your signals you would probably aim to do so in a way that was economical- i.e. only just meets the headway requirement. However you would also need to decide how to protect junctions and this may well imply that you need closer signals that the headway itseld demands. Similarly where you have a station / other slow speed area, it is sensible to make the sections in that area shorter if the stopping requirement looks at all "tight" to achieve.

By now you have probably placed in indicative positions little arrows on your track diagram representing the places in which your first guess is that you'd probably put signals. That is the time to do your stopping calculations; I'd work from my embryonic solution, look for the worst vase (there my be several staions, certainly two directions) and prove that the worst one would deliver the requirement. Note that I am not expecting to be asked what is the best stopping headway that could be achieved by signals at minimum spacing? or even what i the value of the headway that your signalling solution achieves?. If the task is just to provide a compliant solution, them my approach would be to work from my solution to check it works. If I find that it doesn't, then I need to rethink my initial signal positions by closing the spacing more or decide to put in a stretch of 4 aspects in what is otherwise 3 aspects. However, provided your first guess is right (that's where the inital comparison of the two headways comes in useful), it is quicker and easier since just doing calculations from "known" positions; also give more realistic result where signals are spaced at greater than minimum spacing as I assume that drivers start braking at the first caution and brake evenly to come to a stand within that entire distance (i.e. at an average deceleration rather less than the max value specified in the question). This confirms to me that I have still met the requirement if I felt I could get away with widely spaced signals; obviously if I assessed that the NS requirement was onerous then even if the rest of the line had signals spaced generally at 130% braking then I'd have closed the spacing around the station to be much closer to the minimum.

I have had a chance to compare the workings of two other students on this forum; one by "interesting signals" which is 3 aspect and the other by "douglas" which is 4 aspect. Both after some reworking and your comments give almost identical headways of about 244 secs. I am trying to determine which is operationally superior. Will let you know.
Regards
Alex

Look at the comparison graph I did. For simplicity this compares 3 aspects spaced at 2000m with 4 aspects spaced at 1000m. The first caution to instruct the driver to brake falls in the same location (obviously when you think about it!). Hence the difference is that
a) in the 4 aspect case the previous signal is 1000m earlier, whereas i
b)n the 3 aspect case it is 2000m earlier
Therefore the 4 aspects have achieved the 2nd train being 1000m closer.

However suppose the option was more realistically between 3 aspects signals spaced at 1000m and 4 aspects spaced at 1300m.
In the diagram the 3 aspect signal (301) two in rear of platform starter (307) would be at 4000m from it.
The 4 aspect signal (401) three in rear of the platform starter (407) would be at 3900m and hence effectively in the same position (i.e. 403 wouldn't fall at same position as 303 being 600m further away and 401 is 1300m prior to this); therefore giving no significant advantage in terms of distance. Indeed it has probably made things noticeably worse since a driver would brake on seeing 403 at YY, 600m earlier than they would have done when seeing 303 at Y.

Where 4 aspects do score though, is by limiting the impact of a small perturbation of train service. The fact that there are two warnings of the red ansd the signals are closer does mean that if train 1 is slightly late away and train 2 getsa restictive aspect and has to brake, there is a chance to give updated information as soon as the next signal comes into view and, if that signal is by now showing YY, then the driver knows that they can ease off the brakes having already reduced their speed. Conversely in the 3 aspect case they would still have to be braking until the following signal comes into view and, since they have been expecting to stop at it, will be travelling quite slowly. It is things like this that give you "hidden contingency", so I suppose one way of reflecting that numerically is to insist that there is a 10% contingency margin if using 3 aspects but waive that need in the 4 aspect case.

[interesting_signal]

It's taken me a while to get back to this but I've finished my attempt at signalling this layout.

Any comments would be appreciated. Comments would particularly be welcome on the single lines section as I'm a bit unsure on that, in particular when the train comes out of the single line section. I'm not sure that signal 132 is correct. Do I need another signal before the junction in order to warn the driver that signal 132 is at red. Not sure what driver's expectation is when they come to the end of the single line section.

And with buffer stops when do you show a red light, or when do you show a white light or when is it appropriate to show no light at all?

Another issue I'm unsure of is the shunting/reversing of passenger trains. Should my signal 1004 be a main signal (for the purpose of shunting/reversing passenger trains) or is a PL acceptable? Then would 1003 LOS need to be a fixed red too?

[PJW]

It's taken me a while to get back to this but I've finished my attempt at signalling this layout.

Any comments would be appreciated. Comments would particularly be welcome on the single lines section as I'm a bit unsure on that, in particular when the train comes out of the single line section. I'm not sure that signal 132 is correct. Do I need another signal before the junction in order to warn the driver that signal 132 is at red. Not sure what driver's expectation is when they come to the end of the single line section.

And with buffer stops when do you show a red light, or when do you show a white light or when is it appropriate to show no light at all?

Another issue I'm unsure of is the shunting/reversing of passenger trains. Should my signal 1004 be a main signal (for the purpose of shunting/reversing passenger trains) or is a PL acceptable? Then would 1003 LOS need to be a fixed red too?

The scan of the layout only appears to be the central portion showing junctions C and E; perhaps you may need to scan the long layout in several separate files. Hence I don

[interesting_signal]

[quote='PJW' pid='1783' dateline='1279610386']
The scan of the layout only appears to be the central portion showing junctions C and E; perhaps you may need to scan the long layout in several separate files. Hence I don

[PJW]

with buffer stops when do you show a red light, or when do you show a white light or when is it appropriate to show no light at all?

Buffer stops should be red EXCEPT where the red light by night could be misinterpreted by a driver as applying to them; note that a red light exhibited in the cess could be a hand danger signal shown by a member of staff from a position of safety.

Hence where a driver would see the buffers as a red light facing them only one track width away from their line, then the buffer stop light should be white; typically this could be where there is a facing connection into a siding on the left hand side of the line, or a head-shunt from a siding that terminates in a spur. On a double track line, a siding in the opposite cess is not in generally perceived to be a problem as it is far enough away, although it is possible that line curvature could give a problem. Similarly a passenger platform buffer stop even very close to the line is not normally liable to be confused due to the presence of lighting and other visual clues in the area; hence these are almost invariably red.

I would expect that all buffers shown on a signalling plan would have lights, although perhaps those from the running line may not be lit. Note however that the lights aren't PROVED and, except in rare cases, are not fed from a signalling power supply. Historically they would have been parafin lamps; nowadays they are typically fed from a station power supply. We do still show them on a Signalling Plan where they are on or adjacent to the signalled railway.

[PJW]

Comments would particularly be welcome on the single lines section as I'm a bit unsure on that, in particular when the train comes out of the single line section. I'm not sure that signal 132 is correct. Do I need another signal before the junction in order to warn the driver that signal 132 is at red. Not sure what driver's expectation is when they come to the end of the single line section.

Regarding the single line, as you suspected you haven

[PJW]

Another issue I'm unsure of is the shunting/reversing of passenger trains. Should my signal 1004 be a main signal (for the purpose of shunting/reversing passenger trains) or is a PL acceptable? Then would 1003 LOS need to be a fixed red too?

To modern standards any signal that authorises a right-away running aspect for a train should be a main signal; in the past the railway economised by only providing a GPL (or exceptionally an elevated PL) as the exit signal from goods sidings if there was not a large number of train movements (e.g. typically under a dozen per week).

Your question particularly asked re provision of routes for the moving of passenger trains around the layout. Where a main running move exists to route the train in the right direction, then this suffices and no specific shunting route is provided; i.e. there is no need nowadays to provide a PL aspect on a platform starter to inform the driver that they are required to travel only sufficient distance to clear a set of points and get behind a GPL in order to be routed back towards the station into a different platform. The driver is expected to know the move they are going to make and not continue as far as the main route actually gives authority; this is a change from earlier times when there was far less possibility of verbal communication between driver and signaller and hence specific signals were provided so that there was no possible ambiguity re the intended move.
If however there is a need for such a movement to be made where there is no such running move already provided, then the route is provided as a shunt route and therefore display a PL aspect. In many cases therefore, such routes are provided along a running line in the opposite direction for which it is signalled and therefore will be limited by a LOS. Do remember that wherever there is a LOS there will be a move up to it and that this constitutes an opposing move conflicting with the normal running moves along that line; hence any such signal must be a controlled signal rather than an Automatic.

Your specific query related to signal 1004- to be honest I can't see that number but assume that it must be the one at the other end of the Up Platform at station D to 128. A GPL is exactly what you should have here and similarly 1003 is also correct as a LOS. Providing as main signals would just be more expensive and potentially give more potential miseading problems to confuse drivers on the Down Main. Hence you got this correct.

[PJW]

[quote='interesting_signal' pid='1784' dateline='1279626589']
[quote='PJW' pid='1783' dateline='1279610386']
The scan of the layout only appears to be the central portion showing junctions C and E; perhaps you may need to scan the long layout in several separate files. Hence I don

[PJW]

It's taken me a while to get back to this but I've finished my attempt at signalling this layout.
Any comments would be appreciated.

Well done for reading the instructions and permitting a train from the branch to be terminated in either the Bay or the Down Main platform and then reverse. Actually 132 needs relevant route indicator for main route to achieve this but let's assume that this omission was a careless slip.

Arguable whether really need traps 601B, but certainly not wrong- historically bays often had traps because newpaper and parcels vans were often left at the buffer stop ends, but those days have passed and therefore not generally now provided. The Up siding certainly needs them. Just be careful to make obvious that the TCI is on the spur rail deflecting into the cess, not the running rail- due to resolution, I can't be sure what you have drawn.

I don't see the advantage of the ROL between EG and EH; there isn't room for a ROL beyond 127 prior to 601 points that might have been useful, but since you need to lock 601N there is no route possible that would not have been possible with the full overlap over 603. Therefore do not provide.

I am assuming that EH/EJ joint is denoting an overlap beyond 132 adjoinng one beyond 127; reasonably clear but may have been worth a note.

Not yet studed route boxes but don't see why two route indicators associated with PL for 127 and 129. Also since the parallel GPLs also read to the LOS, even though they only have one route I'd have given an "X" route indication as well.

136 should NOT be an Auto because of the moves to the LOS which oppose it; should be a controlled signal with an Auto working facility. You were right to have a track joint beyond the LOS so that any overrun becomes obvious to the signaller- remember that there is no TPWS and that by theri nature there can be a long lengt of train being propelled in front of the locomotive (whereas multiple unit trains would be driven from the cab at that end and then the driver change ends, perhaps by walking through the train, and indeed trains in the Down diretion may be pulled across the facing crossover before then being set back into the Up sSiding, there is always the possibility that one of the freight trains which are to reverse at D may need to make such a move on an odd occasion.

The fixed red 130 is just what is required there; however you have not marked its overlap and I also believe that it should have a joint close to it so that signaller would know if SPADed.
However on the scan I can't see where EE ends and AJ ends so perhaps I am missing something. In fact the AH/AJ joint could well have been the overlap joint beyond 130 as well as for 125- who cares if 130's overlap is 500m long?- it is no disadvantage and don't spend money needlessly.

The question I have though is:
how do you envisage that the freight that runs from A to B is to operate?
From the Down platform I see you have a route for the loco to the LOS on the Up, then normal signals on the Up to junction C in order to get behind 1001 (even though it is further from 505 than it need be) and then back along the Down, but
a) what happens when the loco gets to 125? T
his will be at red because track EE will be occupied by the wagons of the freight train,
b) what would you then do (assuming that you get the loco onto the lefthand end of the train); how does that train proceed to junction C?

I am guessing that you envisage setting back onto the Up Main again and then running under the normal signalling. This is OK but you should really have put a brief note to describe operation.
Did you consider giving 130 a main aspect and providing reversible signalling up to junction C along the Down?
I think perhaps you did because that would seem to be the role of 122 to regain the correct line.

Something isn't tying together; I think that you can see that if you had expllained a bit more then I would be able to give you that much more credit as I could recognise your intention, despite what might be a bit of a silly slip oversight in execution.