2010 Aspect Sequence

[dorothy.pipet]

A new attempt for comments please?
I found this aspect sequence was easier than some other years and was wondering what I had missed, perhaps some suggestions of what "explanation of any assumptions or controls" might be expected please?

[PJW]

You were right to be suspicious; if a question seems too easy then yes you have generally missed something!

This question really focusses on 107 and its routes; the rest of the layout is really just to put that in context I think.

Let's start from the bottom:

107C (C )- you didn't show the PL move into the bay at all; it did say "all routes and classes of routes".......

107C(M)- track BAC looks short to me, appropriate for approach releasing the PL; hence I think the MAR would be (BAB or BAC). Although it is strictly not aspect sequence I think I might have mentioned the track controls applicable to M/C. Certainly should have explained the rationale for the approach release for the slow speed diverging route- WHY it is needed and the LOGIC behind determining what is a suitable release condition.

107B(C )- another route you decided not to do!
Might have been tempted to refer to GK/RT0044 llocking which arguably is aspect sequence.

107B(W)- you made the release BAC for 10secs; as above I was envisaging this a very short track, but since you have declared your assumption that it is 200m long that is absolutely fine; obviously the PL routes would in that case need to have been given a longer time than this route. The problem though is that you have apparently be released from the full overlap yellow!
I must admit that I prefer drawing aspect sequence charts with the Red as the lowest aspect so that the releases tend to "step up" from it, but it doesn't matter- provided that you do connect to the Red. As it is, it appears from your chart that 105 would be showing YY. No!

Again you should have explained WHY a Warning route has a more severe approach release than a diverging route and what determines the time value (as a rough rule of thumb NR specify that the signal is released when the train approaches it at a distance comparable to the overlap length beyond the following signal so for a 100m ROL then the time value on a 200m berth track is set so that it expired for a train decelerating to stop at the signal whilst it is still 100m on the approach).

107B(M)- No issues

107A(M)- This was the biggest problem.
a) You should have shown the MAR (which occurs as "back-up" when the flashing aspect cannot be displayed). Should have explained that approach release must not happen until
1. train passed the signal in rear, and
2. the driver will be able to read (not just see) the PLJI no later than the main aspect. You would then specify the maximum range of 800m but also explain that where there are bridges it can be that the sighting of the PLJI is obscured until train quite close whereas the main aspect, being lower, could be seen considerably further away and hence the need to establish the appropriate place (and therefore track occupation time) at which to release the aspect. Conversely if the whole signal suddenly comes into view simultaneously then there is no need to delay the release beyond the time when the train has passed the previous signal.

b) You should have shown the MAY-FA as a control on 107 (rather than 105). I know that you have "sort of suggested" this by means of Y/YY and Y/G but your representation does not show the step from the initial aspect of Y + JI1- it looks like 105 at FY reads up to 107 at any of those aspects whereas this line should have been dotted with a # reference explaining the approach release condition (the same as that used for the MAR). MAY means signal initially shows Y (plus route indicator!) and then changes to its tru aspect once approaching train sufficiently close.

The control you noted re BAA clear is actually a condition proved initially to ensure that the driver sees a suitable amount of flashing at 105 before passing it so as not to be confused about the aspect that has been observed; however it is a control on signal 107. Once the flashing sequence has been established then it continues even after BAA becomes occupied (for obvious reasons).

You didn't explain why we show FY (and indeed FYY) to a driver when approaching a relatively high speed diverging junction; the problem is that MAR is too restrictive and doesn't allow the advantage of high speed turnouts to be exploited, yet we must ensure that the driver is given information sufficiently early that can regulate the train speed to be appropriate for the divergence rather than being misled in thinking that the junction signal is cleared for its straight route.

Your answer did not make clear that 107 has to decide to "chance its arm" and clear to Y + PLJI in the expectation that 105 will follow its lead and give the require FY in rear, but that it can only remain exposed (i.e. having bypassed the MAR) for a limited time and unless it gets confirmation that its release is being protected by 105 giving the required advance warning to the driver that it must then revert to Red again and wait for the MAR condition to be fulfilled. Similarly if the 105 flasher having started then fails whilst the train is still approaching it, then the junction signal 107 must revert to Red (although in SSI implementation 105 would actually go to red as the only conceivable way of the flashing ceasing would be the TFM going into red retaining, whereas a RRI signal might either be blank or the flasher fail in the energised state and show steady yellow).
Also should have discussed whether it would be necessary, on the layout shown, to have applied the "Colwich control" of demanding 121 off to be able to establish and maintain the flashing sequence.

This is where I think the meat of the marks lay within this question and therefore you wouldn't have scored well on it.

A new attempt for comments please?
I found this aspect sequence was easier than some other years and was wondering what I had missed, perhaps some suggestions of what "explanation of any assumptions or controls" might be expected please?

[asrisaku]

another practice

Thanks, Arnut