Glad that someone is getting stuck into study for the exams; October isn't really that far away- only 5 months to go- and all these Bank Holidays could be useful for that work if you can resist the temptation of spending them more enjoyably.
I haven't actually done this paper myself so just looking at it "cold" on screen to make my comments-
1. You haven't attached your calculations which would have made it easier to comment. However compare the MINIMUM spacing [due to braking requirement] with the MAXIMUM spacing [the more onerous of the two constraints: a) re overbraking, b) headway requirement] and see what tolerance there is between the two extremes. If there isn't much, then that is telling you that although 3 aspects would in theory be a solution, as soon as other constraints such as junction positions, stations etc are taken into account then in reality it'd be too tight to be able to juggle to give a workable solution.
Note that even in circumstances where the headway is easily achieved given signal spacing for braking, there are other constraints. Personally I advise people
not to launch straight into the calculations for this very reason; THINK ABOUT THE SIGNALLING AT CRITICAL PLACES FIRST. In fact you can even do something of this in your 10 minutes reading time, looking at where the constraints on the layout are; afterall the IRSE tend to use the same braking rate each year and there are only a few speeds so having done lots of previous papers you should know pretty much what the braking distance from the relevat speed will be; learn what it is at say 100km/h and that would at least give you a yardstick from which to do a first assessment.
The Distance "Green to Red" method has its uses but does tend to make the consideration purely numeric and doesn't help intuitive understanding. However if you do use it and find that N is only just greater than 2 (i.e. there is only just enough for two signal spacings within the headway distance), then that is telling you that it is tight!
2. I am assuming you are meaning that a train from F towards G (or from G towards F) will be temporarily left in the Up Goods for loco to detach and run around via the Up Main.
I guess that there would be a main signal route from the Up Goods to the Up Main provided as a running move anyway, so the light loco would utilise this. Then certainly a set-back GPL at the trailing end of the crossover would be ok (though if the layout had been slightly different and a main signal for a down direction departure had been needed for that platform at station B then obviously would have been combined with that).
A further GPL would protect trailing crossover at D to signal to a the Up Branch- initially I was thinking of an LOS but actually would be to a signal ainstead (as this line has to be bi-directionally signalled to be able to signal the complete train that way later on!)
Provide a PL associated with the signal on the Up Branch which authorise movement into the loop.
Provide a main signal for departure in the down direction from the loop for the whole train.
However given that there are 6 trains each way, then they'll be 12 runrounds. We need to make (and state!) some assumptions about when these freights run (mainly at night when there are few if any passenger trains but perhaps two runrounds to take place per hour, or very much amongst them in which case it seems as if this runround is hapenning hourly. So whereas signalling above is ok, as you mention it is quite a long way to run just on a PL.
Whereas I feel acceptable from a driver's perspective, I would wish also to consider the effect on line capacityof the move taking place only at say 10mph. If this proved too much of a constraint then providing a main aspect at the right hand end of the platform at B would help; note that this would require a main signal (i.e. fixed red with associated PL aspect) protecting the crossover at D. In fact it may then be sensible to make this a full signal given that the next one has to be a main signal; that would then give an option for a passenger train to reverse at B at not uch more additional cost. If that is the way in which your thought process goes then YOU MUST EXPLAIN since it will look oversignalled by providing a facility not required by the operating specification.
So if you decide that the runround has to be done by main aspects to avoid impacting upon layouts capacity then show where this comes from and then point out that the differental cost of a R/Y/G compared to a R+PL is pretty marginal (indeed from a signal structural viewpoint the former is likely to be cheaper!). In the real world a contractor would raise a "Technical Query" to the client and by stating assumptions to the examiner you reflect this in exam conditions. Of course for that facility to be really useful it'd also need to be possible for the return working; this would need a further junction indicator and route across the trailing crossover at D, so I would not show these but place a note against signal profile and route box "subject to agreement by client"- these "nice-to-haves" all add cost and complexity and so add up even if each individually is "marginal"......
3. Not quite sure what you are meaning; as I think you know "stop and distant" is still 3 aspect signalling (albeit "isolated 3 aspect signalling), even though each of the signals are only 2 aspect signals.
However I think the answer to your question is
"nearly but not quite right"
Certainly there is only one train at any time on the branch doing an out-and-back; given the layout at B it would seem to need to be the same actual train going back onto the branch, rather than "one off and onother on". Hence need minimum number of signals.
Since you have decided to utilise Track Circuit Block (albeit with axle counters), yes you need a signal as a platform starter at C and yes it would be a Red/Green (I think, but see later).
You'd need another signal protecting the diamond which would be a Red/ Yellow and a further one at or beyond the lefthand end of the platform at B which as you say would be a fixed Red.
There would need to be a distant approaching the signal protecting B- this could be provided as a reflectorised distant board. Work out where it would be placed and if it is actually pretty close to C then perhaps reconsider what I said above and make that a 3 aspect signal instead of providing that distant board.
I think the above reflects what you were intending, but not 100% sure from your description.
In the opposite direction, yes the 4 aspect signal would have a route onto the branch, but it would only be capable of showing Green for that direction.
There would however also need to be a reflectorised distant board, braking distance from C; a yellow at the B platform starter would be vastly overbraked and forgotten about by the time the train approaching C.
Also don't forget the bufferstop light at C
Further you ought to make some reference to TPWS for this bufferstop and indeed for your fixed red at B; I hope that you'd be writing some general note re provision of AWS / TPWS applicable to your layout rather than drawing them each time, but you ought to make sure that you amended that wording to incorporate these special cases or draw in explicitly
See also
the relevant post in Attempted Layouts section
(22-04-2011, 09:40 PM)Sid G Wrote: I have been practising attempting past papers and have the following questions arising from attempting the 2009 Main Line layout:
1.Is there a way my calculations could have forewarned me as I would have run into into serious time constraints in the actual exam having started using 3 aspect?
2.For the run around move between F and G;considering the distance, is it ok to signal using shunt moves on the Up Goods considering the distance?
3.Is it ok to signal the line from Station C to Station B using 3 aspect in the C to B direction with a stop and distant signal before the fixed diamond junction and permanent stop signal train length beyond platform for terminating trains.From B to C;use the platform 4 aspect starter to signal all the way to the buffer stop at B.Assuming axle counters for track sections.
