alexgoei Wrote:2005 Part B Q1 aspect sequence chart.
I would appreciate it if you can go through the answer and let me have your comments please.
I have annotated your diagram and included as attachement to complement the text below.
Overall pretty reasonable, but:
a. Where a route is approached released (MAR) and tou have correclt shown the "trip" condition (and have followed the convention of a dashed vertical line to make this clearer) you have also connected the aspect sequence line back to a higher aspect on the signal in rear! That is to miss the whole point- the junction signal is being held at red precisely so that the approaching train gets restrictive aspects on the approach; otherwise they would sudddenly find they need to traverse the diverging junction and are travelling too fast to be able to brake to a safe speed. [Not sure why you seemed to do this consistently on this layout as I believe OK on previous papers you have done]
b. You missed approach release on 158 entirely. Perhaps it was because you didn't see the route box that is not in the most obvious place on the plan; however the junction speeds should have made you recognise necessity anyway.
c. From 3 aspect signals reading onto 4 aspect signals, you failed to tell signals 144/ 146 what to display when 114 at Yellow (similarly others). Working back from the red toward the approaching train should have made you realisethat there are 3 signals which can approach 114 but you only had drawn 1 aspect sequence line to its Yellow. Generally the 3 aspect signal should show Green up to Yellow BUT check that a driver encountering the 4 aspect at Yellow (thus without seeing a Double Yellow) will actually have enough braking distance to the Red.
d. You did the right thing by pointing out the "deliberate" error on the plan; there normally are one or two. I am still undecided whether it is in fact deliberate ploy by examiners or is actually the result of inadequate checking on their behalf; the fact is that they are far from unusal and candidates must learn to cope with them- not unknown in the real world of course.
e. Not an error as such but the plan has deliberately not provided "better aspects" in places where they would be of little or no benefit. In current day with computer interlockings if the signal has aspects then we use them almost "comes for free"; in route relay and lineside cabling installations there was real cost so if no value in providing, then don't. The examiners still live in that world. Candidates are told to "answer to their practices", so I think it eminently reasonable to show the better aspects although I'd recommend a note that "provided over and above route box info in accordance with modern practice for such situations". In the case of 146 to 118 you can make the problem go away by stating "assume 118 is always approach released"- a very reasonable practice tha we might still do. [You just left the Y and G unconnected without explanantion. Suggest never do so; if a signal is from bay platform etc then it is correct but still put a note on the diagram to explain it "cannot be approached by a main aspect signalled move".] Always MAR however would not really be the solution for 108 as this provides the only running move from the Down to the Up. On this one I have followed the plan so 114 shows "Y with pos 4" whatever the displayed aspect at 108 even though nowadays we wouldn't like to do this (it's very likely that driver could see 108 prior to passing 114); the modern alternative would to have made use of "G with pos 4" when 108 displaying any proceed aspect and added the norte of explanation as described above.
Hence two main "sins"
1. Showing unrestrictive aspect on signal in rear when junction signal set for divergence significantly less than straight route speed
2. Not ensuring that every signal in rear has its aspect uniquely defined for every possible aspect of the signal in advance.
