I thought it was very good; you chose 3 examples of definite interlocking releases (whereas approach release of an aspect would be arguable depending on quite one's definition of "interlocking") and gave one page on each; hence appropriate length and equal treatment as the marks suggest.
If being critical I might say that you spent more time than necessary re the imposition of A/L but I can't see there was much more to say about the releases so I don't think I'd have done much differently.
The one thing I think you should definitely should have mentioned was the
rationale for the time depending on the distance of the signal spacing (as opposed to the distance from the first caution to the red in 4-aspect territory) and how those times were determined, particularly why the
time value is different for mixed traffic and all passenger railway.
Basically it is a judgement of how slowly a train could be going having passed the previous signal just prior to its reversion and yet still be moving too fast to be able to stop when unexpectedly encountering the signal that had in the meantime been replaced to red. In these circumstances the driver will only become aware of its status once within that signal's sighting distance, so it is then a question of whether the train would be likely to stop within that length. Since freight trains have worse brakes than passenger trains (particularly in terms of the brake build-up time since this is propagated by air pressure passing down the long train rather than virtually instantaneously by electricity), then to be able to do so their initial speed must be lower and hence the A/L timer has to allow a longer time to traverse the section if there is the possibility of freight trains on that line.
Actually of course in a long section it would be possible for a passenger to have come to a stand for some reason and then still have plenty of length to accelerate again to be going too fast to then be able to stop on sight, the A/L timer long since having expired, so it is only a balance of probability, not a guarantee!
In fact I remember test logging a site where a signal approached by a low line speed of about 15mph where the A/L timer was basically useless- a train running at the full permitted speed would not arrive at the signal before its A/L had timed out if had been replaced once train past the previous signal, so there was no sense in providing a 180sec timer- it could just as well have been 60sec since the only real value was for circumstances in which a train was on the immediate approach when it was reverted.
You could also have added some bullets re-
a) MAR routes
b) the additional release sometimes provided if signal replaced whilst berth TC occupied
c) signals from sidings and terminal stations etc.
For the other sections it may have been worth adding a little more re the rationale behind the table of values to explain the philosophy behind the "time to a stand" values, rather than the "it is in the standard" sort of response you gave. Hence knowing that approx 30sec is used for a 200m track suggests that the average speed over that length is taken to be 200/30= 7m/s = 15mph and that the shortest time for a track of some 50m is around 15sec = 7mph allows you to display a little more knowledge and hint at how these figures may have been established.
Indeed in some scenario they actually are not long enough- the speed may be so low that there is very little chance of a SPAD happening but if these values are used by the interlocking when trains are provided with ETCS, then if the driver is coming to a stand very defensively what will happen is that the interlocking will release the overlap and therefore the SvL (Supervised Location = safe distance) advised to the train will be pulled back from the end of the overlap to the signal itself- when the train receives that update then there is a chance of it causing an Emergency Brake application as it needs a margin between the place where a driver is supposed to stop the train and where it must guarantee to do so in the event of a SPAD.
You could also have noted that there are interlockings (such as SIMIS-W) that reset the timer to zero when setting the route in rear) so that even in the event of a right-side track failure there is some element of time delay imposed and also enables the timer to be "down-proved" prior to it being potentially needed later to give a release.
You could also have mentioned the fact that cognisance has to be taken of the technology implementing the time values. It is important that in none of these examples that the time should be "short" and therefore in failure conditions it should become longer or infinite. Depending on the place in the cycle, SSI implemented timers can vary by up to 2 sec from their nominal value; hence where this is judged critical the time value in the data has to be different from the theoretical one.
Thermal timers need to be adjusted when they are truly cold; once they have been used they start off warm and if used again soon afterwards will take rather longer to time. Obviously if the tester then adjusts and has multiple attempts at timing to get them timing just right, when the are actually used for real then they will probably time out too early as they will generally start cold.
However these are just a few suggestions of the sort of things that you could have included in your answer to extend it a little further; however I do think it was well answered, with the exception of giving a bit more of a rationale about the times selected. I think it would have been at least a Credit anyway.
P.S. Is it a reasonable assumption that your train journey is only 15 minutes each way?
(01-09-2013, 02:23 PM)dorothy.pipet Wrote: An attempt at this question - written during 2 train journeys so probably not my best work;
Scanner fail: the last line in the 2nd page should continue "tracks, and the value based on the combined length"
I rather think my explanation of Approach locking release timer (p1) - what it depends on - is weak so if someone can help me out there as well as general comments, I'd be grateful.
