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1994 Q4 Automatic Level Crossing- Interface with Signalling
#1
I have attempted Q4 from the 1994 paper (also in the Mod 3 study pack - section on level crossings)

The question was:
An automatic level crossing requires modernising because development of the surrounding area has given it significant commuter road usage. The crossing is situated 100 metres (110 yards) from a station platform, at which alternate trains stop. Describe the interface between the crossing and the signalling, for a bi-directionally signalled double line without pointwork.

I'd welcome any comments, particularly on whether I've even chosen the right type of crossing for this situation, and whether I've taken the right/best approach to the question. I realise now as I submit this that a diagram at the beginning probably would have helped, but I'm just submitting my answer as it stands.
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#2
(29-01-2011, 06:53 PM)interesting_signal Wrote: I have attempted Q4 from the 1994 paper (also in the Mod 3 study pack - section on level crossings)

The question was:
An automatic level crossing requires modernising because development of the surrounding area has given it significant commuter road usage. The crossing is situated 100 metres (110 yards) from a station platform, at which alternate trains stop. Describe the interface between the crossing and the signalling, for a bi-directionally signalled double line without pointwork.

I'd welcome any comments, particularly on whether I've even chosen the right type of crossing for this situation, and whether I've taken the right/best approach to the question. I realise now as I submit this that a diagram at the beginning probably would have helped, but I'm just submitting my answer as it stands.

First let me say that I am pleased to have a mod 3 written question attempted; from what we hear it seems sensible that students concentrate on preparation for them and ideed there is a pretty comprehensive series of attempts for Control Tables over recent years, with exception of 2010 of course. If we start getting significant numbers posted, then I'll rearrange either "per year" or "per topic" as for some of the other areas of the Forum.
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I am myself a little confused with the wording of the question; I do not feel that I am correctly getting the significance of the opening sentence. It is almost as if the increased commuter road traffic should be driving a consideration of the type of crossing rather than a pure upgrade.

1. We could assume that it was currently an AHBC and that there was now an increased risk of tailback over the crossing and therefore becoming dangerous if road user abuse; 10 seconds isn't much road open time and the drivers of vehicles that have been stopped in a queue could be forgiven for just following the car ahead without having looked too much at the state of the road on the other side of the railway- an oncoming train however will NOT be forgiving if they are still on the crossing having been unable to exit it. The wording of the remainder of the question doesn't suggest that one should be describing a CCTV level crossing (that would be the only feasible upgrade) and I'd be reluctant to do so given the clear description of an "automatic level crossing". Similarly whilst undoubtedly safer, what would conversion mean for that commuter traffic other than considerably more delay, so the solution just has to be a bridge which isn't really an appropriate answer in a mod3 question (but may have been to a somewhat similar question posed in mod 1).

2. Perhaps we could assume that it was originally a very lightly used road, but that at commuter time traffic is significant but still moderate. This would fit rather better. We could postulate that the crossing wasn't originally AHBC but will be upgraded to be so or that it always has been an AHBC and is to be renewed more or less "like for like". In the latter case then I am back where I started- what's the relevance of much of the first sentence of the question?

In the former case need to conider what sort of crossing it was before; an AOC(L) doesn't seem likely on a double track and if there is any reasonable line speed, particularly as not all trains stop at the station as visibility would be imposing a relatively low crossing speed. I suppose that in 1994 there may have been (haven't checked the history) a significant number of AOC®s around and perhaps that was the intention of the question to say convert such a crossing by providing it with barriers (in the same way that AOC(L)s are gradually being converted to ABC(L)s nowadays. Exercise for student- find out when the Stott Report was published, data and the aftermath of Lockington accident ; reckon that it was late 1980s, but these things do take time to percolate before implementation programmes get underway and widely known, so perhaps this question was topical within the UK at the time of the exam.
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Hence I'd be a little hesitant at tackling the exam, knowing that I couldn't quite tune-in to the examiners' wavelength. My comments therefore are on the basis of how I feel your answer addresses the remainder of the question.
I suppose if I had decided to do it, then I would state an initial assumption that even the increased commuter traffic was such that there was "no blocking back risk" and therefore having risk-assessed the existing AHBC that the upgrade was primarily a like for like renewal, but to the latest standards (in those days quartz halogen road lights, but with the lamp proving relays that lamp proved "one lamp per post" rather than "one lamp per side of railway"; also bi-directional strike in for double track even if only signalled for one direction etc.) but without much obvious change to the road user.

The question that I am considering is therefore:
Describe the INTERFACE between the CROSSING and the SIGNALLING for an AHBC where stopping controls are provided for trains stopping at a station 100m away.

I think that I'd have interpreted this as
a) "the crossing" being a standalone REB with all the crossing circuitry within it and interfacing to its "own" trackside kit as an almost freestanding entity, and
b) "the signalling" being everything else that would be there anyway if the crossing hadn't existed and may well be an SSI / remote control centre arrangement.
This gives me a very clear "interface", being everything that the one part needs to tell / learn from the other part in order that "the right hand knows what the left hand is doing" and work satisfactorily together as a combined system.

Hence the perfectly standard "level crossing stuff" is not applicable to this definition of the question; I think that reading your answer there was too much of this- see comments below re how to dress up differently and therefore be able to incorporate.

Although I have used the split of technologies of RRI for the LX and SSI for the signalling to give a nice clear definition of the boundary and thus interface, the same would be true within an area of existing RRI signalling where shall we say that a AHBC was implemented in a series of lineside locs mainly for it but also feeding other signalling and telecomms equipment in the area.
A renewal / upgrade project may well then provide a new REB to take over the functionality and some or all of the old locs may well have to be retained but with th AHBC related equipment removed from them. I suppose this creates an interface even within an RRI solution and may be why the question was worded the way it was but it is only after I got to the end of the question that this penny dropped. Indeed even in a completely New Works RRI job there is an interface to consider, but if it is the same technology in the same room then precisely what is the one and what id the other becomes more of a grey area and frankly a distinction not worth making.


As ever it depends on defining "the system boundary". I'd tackle a bit like this:

a) The crossing obviously needs to know when a train may be approaching and the track circuit information will be known by the SSI via its track circuit (and indeed treadle) inputs, so this is certainly an interface that is to be defined and it is reasonable therefore to talk about the placement of the strike-in points which the crossing requires. Also that these strike-in must be placed to give 10 seconds additional to the nominal 27 seconds in order to provide for the "Minimum Road Open Time".

b) The crossing needs to know whether the signaller has selected "stopping train" and again this needs to come across the interface. Consideration of how the combined system is known to work in a safe manner in this mode is also pertinent; if the crossing strike-in is being delayed on the basis of this selection, how can we be sure that adequate warning time will still be given- what assurance have we got that the train really will be travelling at the reduced speed that the closer strike in is assuming?

c) Related to this is the information from the crossing to the signalling that it has actually responded to the call to operate and thus any signal that has been held to danger in order to protect it (as a result of the stopping control negating the normal strike-in) has delayed clearance imposed to prevent a train at or approaching it from being able to reach the crossing too soon.

d) The signaller (if not the SSI) must have assurance that the crossing is healthy and would operate when demanded together with an indication that it has indeed done so; also that it has remained down for an excessive time and thus closing the road, perhaps necessarily to protect a failure but still undesirably.

I think it is reasonable to assume (but should state) that the crossing itself establishes "directionality of movement" by looking at its own local inputs from the local track circuits and treadles in the immediate vicinity of the road, rather than any input from the remainder of the signalling

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So now looking at your answer.
I think that you got the key information and didn't write any rubbish so that is certainly a good start. I think you did stray a little into areas that were unnecessary, rather than purely concentrating on the INTERFACE. It did read a bit like a general description of such a crossing rather than specifically targetted to the actual question.
The discussion re the lamp proving arrangements of the wig-wags really not relevant- a more general reference to it as being one of the constituents that are summated into the general crossing OK/ Fault input that is displayed to the signaller and therefore an interface function would have made it relevant; indeed there are many things in that circuit and you could have displayed your knowledge re crossings by listing. I read the section re the LCU and thought "true but not relevant to question- padding" whereas had you simply said that the LCU door is proved closed and switch "not turned away from auto" (this proving that local control of the crossing has not been seized) are included in the interface circuit that feed the signallers alarm, I'd have thought "highly relevant and a good point that most candidates would overlook- therefore mark well accordingly".

The lesson to learn from this is that you should not just be putting down your nuggets of knowledge, but actually [i]displaying them to best advantage[i]; like in a shop people are more likely to buy a promoted item in a specific display than if they have to go look for it amongst lots of other stuff they don't want. In the exam you are selling your wares, so part of it is about presentation rather than just the basic product .

Read through your answer again and see how you could have made each paragraph more relevant to what was actually asked- in most cases you can cut some of the detail but add a line or so that explains how it is an interface issue. Perhaps the audible warning isn't relevant at all- perhaps though you can argue that the "Another Train Coming" is and as a result the frequency and pitch of the audible warning is changed.

Overall I don't think it was a bad answer- it should certainly Pass even though only two sides and looking a litle short.
A diagram perhaps might have been useful but could well have made your answer even more like an "I am giving you a description of AHBC that I have previously learnt".
However if you had shown
a) the strike in tracks feeding into SSI TFMs and then
b) a SSI TFM output feeding into the crossing REB,
c) the signals fed from SSI but
d) the crossing lights, barriers, local crossing track and treadles around the crossing fed into the REB and
e) the functions such as Barriers Up / Working / Failed and Power Supply Normal / Standby feeding from REB into SSI TFM then the diagram would so have lifted your answer that together with some tweaks to the wording as suggested above then I am sure it would have been a Credit.

Add a bit more detail re
a) how the "Stopping" functionality affects the signalling, and
b) recognise that in the direction having a platform starter signal
i) the initiation of the crossing is not purely track occupancy but is conditioned on the route being set and signal ready to clear and thus a summation SSI output to the REB,
ii) the SSI then needing feedback from the crossing that it has initiated before commencing a timer (whose value is determined by the difference between crossing warning time and the fastest attainable time for a train to reach the crossing having either already been stood at it or alternatively may still be moving on the approach to the signal) that then permits the signal to clear
c) the arrangements for permitting the level crossing to raise should it have been activated by an approaching train which then does not proceed having stopped in rear of a signal within the strike-in whose Approach Locking has timed out
then I think it would have been a Distinction answer.

IF OF COURSE I HAVE UNDERSTOOD WHAT EXAMINERS WERE EXPECTING- I am still hesitant re that opening gambit.......

I'd be very interested if anyone else feels they fully understood the question and let me know the significance of the upgrade and the increased commuter traffic that has passed me by.
THOUGHTS?
PJW
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#3
I did have to read the question a number of times before putting pen to paper, and wasn't sure if I had interpreted it correctly either. Level crossing 'upgrades' which I've come across in my job have usually been upgrading from an open crossing to a barrier crossing. I took the intention of the question to be along the lines of 'since there is heavier traffic, an open crossing is longer suitable (higher risk), so the crossing should be upgraded to a barrier crossing. Although there is not much to go on as to what type the existing crossing is. As you said it could already have been a barrier crossing.

I took the interface between the crossing and the signalling to be a broad definition, hence including things like wig-wags. I tried not do write just a 'brain dump' of everything about AHBC, but did feel at the end, it had sort of ended up a bit like that.

I probably wouldn't attempt this type of question in the exam either, because it took awhile to read (and re-read) and try interpret the meaning. Therefore I would have wasted precious time in the exam.

Re the Stott Report , it was published towards the end of 1987, and the first ABCL commissioned was in 1991. So not sure that explains the question. Either way I am just using it as a learning exercise. Currently working my way through the level crossing questions in the study pack (1994 onwards) so you should see some more attempts on this topic from me soon.

Thanks for the feedback.
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