Answer started very well when considering the track circuit constraints. Less good when considering the effects of signalling layout design, particularly re linespeed braking and headway.
Gets better again towrds the end when thinking about interlocking controls.
Easy to understand why this would be the case from knowledge of your design role and the fact the company specialises in NR GRIP stages 5-8. Obviously the aim of IRSE Exam is to extend students' knowledge beyond their "day job"- hence I have "gone to town" a bit in an attempt to help you fill some apparent gaps.
Wheelbase-
Treated well, diagram essential; I think you should have explained just a little more about "sequential proving" to demonstrate that you really understood rather than quoting a buzz-word: for a train moving from left to right once AB has become occupied then it cannot be regarded as being clear again unless AA is also clear. Otherwise fine.
Overhang
Fine; diagram essential. By naming the tracks here would have allowed you to explain a little more re what including a foul track within the interlocking controls of the point really entails.
Overall length
You didn't really seem to respond to this element apart form a slight reference to effect on headway (that probably would be quite slight in reality) but actually you did have somerelevant info at the end of your answer under the heading of "stations and junctions". The examiner ought to find and give you the relevant marks; however as a candidate you'd be running the risk taht being human they could overlook- so from that perspective better to make it more obviously addressing the element of the question.
The key issues here I think are involving standage-
a) when stopped at a signal, is the back of the longer train now trailing back across a level crossing, blocking a crittical junction and not permitting other routes to be set that used to be possible?;
b) is the distance a train can stand back from a signal to give the driver good sighting being compromised because of the need to get the train fully into the platform for example?
May particularly be the case where trains terminate; tif train stops a suitable distance prior to the signal at end of its route, the driver then changes end and finds the cab is too close to the opposite direction signal that not able to see it from the cab due to sightlines etc. When evaluating such things, also consider the case say of a 10 car train that is split into two 5 car sets to be utilised eparately- the separation distance would eat further into available viewing distance.
c) can had a really serious effect on following headway in one type of circumstance. eSuppose that the original signalling was designed to maximise headway- this wouldd have ensured that existing trains stood at a signal (perhaps a platform starter) just do not occupy the track which in the overlap of the signal in rear because of careful placement of that track joint close behind the train. The new trains however would occupy this section; since they are still in the overlap, then there will be two rather than one signal held to danger behind them. A critical few metres could approximately halve the line capacity!
d) Will the train fit in the staion platform at all / where are the doors- effect on disabled access, selective door opening etc?
e) are there instances on the layout where the train is so long that it still hasn't released the approach locking o the siganl in rear when waiting at the exit signal and therefore the overlap ahead of it is unable to time out, thus restricting the use of the layout and potentially leading to a "Mexican standoff" where two trains are safely separated but each blocking the other's forward progress?
f)Lime Street Control- train length measuring.
Acceleration
I think that I may well have, like you, treated as part of braking or perhaps after that section. Many of the issues are similar, relating to "attainable speed".
I would however included some specific info re acceleration; the most obvious to me is when calculating the warning time given to a level crossing (or track worker warning system or similar) where there is a signal relatively close to it and strike-in inhibited by signal route not set.
There is a hazard that if the route is subsequently set, the train can start from the signal and reach the crossing after too short a time for adequate warning and hence there is a need for delayed clearance control. A better accelerating train could necessitate a longer time delay or provision of a delay on a signal that didn't previously need it.
Braking
First note that the question you quoted stated braking distance HIGHER whereas you seem to have answered as if it had said LOWER; to be honest I would have assumed that modrn rolling stock would have had a higher braking RATE. Anyway in the text below I have followed your lead.
Beware comment that from the same linespeed (note this is a useful concept but not a term that NR now officially recognise!) that trains with better braking can leave braking later. Certainly true for approaching either a PSR or TSR, but less so when approaching a signal at red. It will be different when there is ETCS but for lineside signaling, driver is required to brake and substantially reduce train speed on sighting the first cautionary aspect. the effect for following train headway re better braking is therefore not very great.
Given that the line is signaled for existing trains, presumably some of which will continue to operate, then there is no need to worry about "excessive braking"; basically the train driver will become used to the fact that this class of train has better braking than the previous standard and therefore will experience this factor for the entire route over which drives; it will require a slightly different braking technique and liable to coast at an intermediate speed for longer prior to applying final braking to a stand, but to a large extent will drive similarly to other trains, some of which the driver may still be driving over that same route.
Your answer did state that particularly important to assess where there are differential speed restrictions, but you didn't make much of that. the point is that given the new trains have better braking, then there will be a desire for them to operate over the route at a higher speed- indeed this is why the historic concept of linespeed is not as useful as once it was. This speed profile will need to be determined, governed by such things as the civil's speed limits at any place (axle loads on structures, dynamic profile of vehicle, cant deficiency on curves etc), the braking being compatible with the existing signal spacing etc., the drivability of the route (i.e. not a ridiculous number of different speed limits for very short stretches) etc. So actually this tends also to solve any concerns due to overbraking; one defines an Enhanced Permissible Speed that is higher than the original linespeed for which the signaling was designed in such a way that the trains can exploit the potential for faster journeys to the full and therefore inevitably "use up the slack" that would otherwise have existed between the signal spacing that pre-existed and the braking distance they actually need.
If the line was resignalled at the time of fleet introduction and this train type had become the dominant user, then the signaling and the continuous signing of the linespeed would have been designed for their speed profile; a lower "differential speed" would be applied to make the signal spacing optimized for them to be safely usable for trains of less braking performance.
Hence this section of your answer was broadly along the right lines but expressed in a way that suggests to me that you hadn't quite appreciated the nuance of the situation and you were tending to concentrate on the wrong perspective.
Perhaps the most important point to make about the increased braking rate is that in some circumstances it can actually make the situation less safe. Existing signalling may have been based on a number of assumptions that are no longer true; in particular the concept of "attainable speed". A linespeed profile tends to abruptly change at certain positions, whereas we know that any train cannot change speed instantaneously and this might just have been factored into previous assessments- a hidden assumption.
Suppose on a 100mph line there is a section though a main station at 40mph; there will be some signals on the nominally 100mph section where one knows that a train must be going much slower than that or else it won't be able to comply with the forthcoming restriction. This perhaps may have been used in the past to justify sub-standard braking, limited sighting distances or strike in points for level crossings etc; however once a train with better braking exists then its driver will legitimately be able to leave braking later and thus may pass one of the intermediate signals at rather higher a speed than would have been thought possible given those facts known at time of signalling design.
Same would apply if a line previously signalled for 100mph can later be utilised at 125mph beciause of introduction of trains with better braking and so can stop in the existing distance between the double yellow and the red; the problem is that they will be passing the single yellow very much faster than was previously the case- don't forget that energy is propostional to the square of the speed, so the speed changes only slightly during the early stages of braking. In particular a form of junction approach release (originally known as "free yellow"; now known as MAY-YY) has become particularly undesirable. It was always based on a "white lie"; we show the driver YY ostensibly to tell them they are to stop in 2 signal sections on the straight route, yet really because we are diverting the train through a medium speed diverging point and we want them to brake a bit, but not too much as they would have to if we made the junction signal as MAR. This "fiddle" could be made to work provided all rolling stock of similar characteristics and the engineers got their sums right; change the rolling stock though and it becomes rather dodgy, increase the initial approach speed on the basis of having better brakes and it could well become distinctly unsafe. One doesn't really know what was justified on what basis- the signal engineers leaving us the legacy did not often state all their assumptions; even if they did write them down would we be sure we found and understood them all? Indeed before the days of disc brakes, there would have been no need for anyone to assume brakes acting on the wheel rim- the more relevant assumption to record at the time would have been how many of the vehicles of a freight train had automatic brakes at all, yet today we take for granted that all have them fitted and, at most, a very low number of them might be isolated. The world has changed. Hence tend to have to re-evaluate everything along the line in the new context.
Certainly should mention evaluation of the TPWS fitment; check that the fitment is adequate for containing an overrun of the new rolling stock within the Safe Overrun Distance. Also that the OSS fitment is still an acceptable compromise between giving confidece of stopping a train liable to SPAD and on the otther hand not giiving rise to false interventions- again this relates to the different speed/ distance curves for the new trains compared to the previous ones.
Obviously I have given a long-winded explanation above in an attempt to explain to someone who did not fully comprehend and recognise the significance; in the exam you the candidate are not attempting to teach the examiner but merely show them nough that they can tell you understand. Hence I would recommend drawing a couple of diagrams, with some brief captioning-
a) difference in speed profile to a non-zero speed from i) trains at the original speed at the original brake rate and ii) trains at a new higher speed at the enhanced brake rate;
b) difference in speed profile when braking to a stand at a signal by i) trains at the original brake rate and ii) trains at the enhanced brake rate
the aim beiong to demonstrate that the speed at any intermediate place can be significantly different from that which the signalling system originally tacitly assumed.
Then just a list of items that could be affected:
1. Minimum Reading Time of signals
2. OSS set speed and position
3. Delayed clearance for signals protecting automatic level crossings
4. Justification of lengths of overlaps, reduced and restriicted overlaps
5. MAY-YY aspect sequence
6. etc etc
I'd then leave a lot of space / continue answer on a fresh page, get to the end and if have time left then return to this section to add detail on as many as possible within time constraint.
Numbered lists are better than bullets therefore in such situations as saves time / repeating since clear to which item the additional info relates.
other elements of your answer
"Overlaps and Protection"
Personally I think it better to organise content in manner suggested by the question rather than as you have. Most of this content is ok, but it could have been expressed better- did you really mean that "length of overlaps is generally related to line speed"- in general on NR one provides
a standard length overlap, although I agree there are some circumstances in which we deviate.
You made interesting point re ATP; yes the train brake rate is held on board and hence is something that will need to be set as a parameter on the trains themselves. The problem will come if the new trains are to travel at difference maximum speeds than the existing rolling stock sinece the lineside equipment holds the speed limit, gradient, distance to next loop, the distance to the next signal at danger, overlap length etc and transmits these to the train. I suppose it would be possible to "bodge" the interpretation of the speed infomation so that the on-board of the new train adjusts the value in some way, but this is fraught as essentially it would be another "white lie" and one would have to be very careful that it was safe in all circumstances and restrictive in as few cases as possible. Difficult- certainly whilst there are still trains of the old variety utilising it for their operation.
"Stations and Junctions"
Full of good stuff, but again wasn't really looking for it here under this heading.
A lot to be said for using sub bullets and hence putting each idea on its own line.
Don't be frightened of using more paper, leaving more white space.
(08-07-2013, 10:55 AM)dorothy.pipet Wrote: An attempt at Q7 from 2007:
It is proposed to introduce a new type of train onto an existing line. The new train differs from those already on the line in the following ways:
* Its wheelbase is longer.
* The distance from the front of the train to the leading axle is longer.
* The overall length of the train is longer.
* Its acceleration rate is higher.
* Its braking distance is higher.
Discuss what factors the signal engineer must assess, and what changes may have to be made to the signalling infrastructure, before the train is permitted to use the line.
I have done this untimed and suspect my answer is longer than I would have time for in the exam. Your comments please?
