(07-04-2010, 10:36 PM)interesting_signal Wrote: Here's my updated answers. Note, I have left out question 9 for now as I haven't studied the stopping calculations yet.
Suggest leave stopping headway calcs for now then; there will be relevant example at Signet.
Q4 additional
To achieve headway requirement you are correct that following trains need to be able to be signalled within 2040m of each other; you then calculated that this meant adjacent block signals should be no more than 1460m apart. You made an error in your conclusion (which I failed to point out) when you stated that distance to be 2040m.
To achieve the braking requirement you correctly state that distant signals should be a minimum of 300m in rear of the block signals. In isolated 3 aspect signalling there would not normally be a reason to place the distant signals much further than this, except perhaps to give better sighting due to track alignment, bridges etc.
If you decided (for the reasons you stated) that you wanted to provide 3 aspect continuous signalling then you'd need to decide what overbraking would be acceptable. If you simply placed the same number of signals evenly (and thus actually provided nearly twice the capacity demanded but at a cost not hugely more than that of the isolated 3 aspect solution) then each signal would be approx 730m apart; that would make the sections some 250% overbraked- you'd need to decide whether you could live with that.
Perhaps you meant provide more signals (i.e an extra one between the positions of the R/Gs and then space them all out evenly) even though this would add significantly to costs- on a simplistic "pure SEU" basis some 25% increase in costs. This would place them at approx 485m intervals which would be some 160% of braking- in percentage terms that still sounds an awful lot but in absolute terms it isn't a huge distance. At the permissible speed it would be approx 24seconds. In terms of sighting, it may well be possible to see one signal from the next- certainly it is not likely to be long before it comes into view- the signals are spaced at not much more than twice the AWS distance apart. I think I would be comfortable with that spacing; however I wouldn't feel it reasonable to have provided a 3 aspect signal every 1460m to provide the bare minimum capacity with each section some 480% braking; you did not really describe what you were proposing when talking about 3 aspect signalling.
Hence you need to think about why there is an "overbraking rule", the risk which it is seeking to mitigate and therefore what would be the consequences of breaking the rule. Perhaps you feel that this is unfair comment since that is what you meant by what you wrote; I do agree that you seemed to mention the right considerations, but really I wasn't fully convinced. Seemed to me that you were quoting what you had picked up from reading rather than demonstrating that you really understood. You would have got credit for what you did write, but not as much as if you had worded more convincingly. So learn from this- whether by improving your understanding or by improving your demonstration that you do understand, because it is the impression you give the examiner that counts.
Conversely I was far happier re your description re Q5- I believe that you understand the issue.
Q8
Fine to start with, determining what the headway time would be for 3 and 4 aspects with signals placed at minimum spacing suitable for the passenger trains (though a little explanation in words like this of what you were calculating would have been advantageous).
Then I wasn't very sure what you were doing- you seemed to derive the HD again from the HT and because of rounding errors the 3678m changed to 3672m. You then seemed to use the DGR method on the figures and almost be surprised that you came to the same answer. You didn't explain what the significance of calculating the signal spacing both by 2025/2 and also 3032/3 was, or indeed what is the significance of the the ratio of DGR/S being 1.5. A little more clarity in what you are calculating, what is a maximum and what is a minimum figure would help.
Re the freight train- note that you have to provide signalling on this line for these two different train types. An easy way of thinking about this is that during the day there is a need to run passenger services at 80mph at 120s timetabled headway, yet during the night the same line and same signalling is to be able to support a train service of freight trains running at 60mph at timetabled headway of 180s.
Which trains have the longest braking distance?- this fixes the MINIMUM signal spacing
which trains have the more onerous headway requirement?- this fixes the MAXIMUM signal spacing
Overall feeling is that you haven't quite seen "the wood for the trees"- the mechanism seems to be whirling away quite nicely, yet the penny didn't quite drop! Hopefully the above will give it a jolt......
As you yourself said, the role of these exercises is to get you to think and really appreciate what it is all about- you are well on the way and once it has truly sunk in you'll not forget it, so persevere!
Q18/19
Yes one error corrected but still one silly!
You are calculating a TIME [T] which as you correctly say is a DISTANCE [L] divided by a VELOCITY [L/T]
Hence all the terms in the brackets on the right of your equation must have the dimensions of LENGTH [L]. However you wrote "Sighting Time"which I imagine was just a silly slip for "Sighting Distance".
Always worth a 2nd look at any equations to make sure that all quantities are in the same units and that the formula is DIMENSIONALLY correct.
PJW