(13-06-2010, 11:05 PM)interesting_signal Wrote: OK, I've spent a bit more time on the stopping calcs and have had a go at drawing the graph for 3-aspect signalling, as per previous my calculations. Apologies in advance that it's a bit messy! Also, had to squash in signal 5, so that part's not quite to scale.
My conclusion was that 3 aspect signalling can meet the stopping headway requirement of 360s. My graphs shows that a headway of 242s can be achieved with this spacing.
Note, the aspect sequence only relates to (is caused by) the first train. I had added these on before I drew the second train. Was going to scribble them out, but concluded it might be less messy to leave them in. Anyways, I've got a better understanding of this now, but need a lot more practice.
Yes, things are certainly coming together.
It is neat enough- there are only two things that I would advise
a) you draw with a rule the "constant speed" to more clearly differentiate this from the "changing speed free-hand" - in particular it loked quite odd for the first train near signal 4!
b) whilst you clearly took the aspect sequence line from the back of the first train clearing 3's overlap to give a G on sig 1, all the other lines seemed to be derived from the FRONT of the train but not necessarily passing an overlap joint- hence it looks confusing and I wasn't quite sure what to make of them, particularly since there seemed to be two different "times" at which the sequence changed to show sig 1@Y, sig2@R about 80 sec apart......
Re the aspect sequence- completely correct that relates to first train- the only thing is that the aspects would of course revert again once the 2nd train passes the relevant signal.
It was fine to leave them just as you did- just note that depicting the sequence as a result of train 1;
for best headway the aim is to place the "curve" for train 2 as low down the paper as possible whilst ensring that its driver gets sufficient sighting time of a signal displaying a sufficiently good aspect that can continue without unnecessary braking; aspect reversions as result of 2nd train not depicted.
I attach a couple of other sketches
a) a similar diagram to yours,
b) one of whch attempts to relate the headway achievable with 3-aspects to that achievable with 4-aspects.
a)We came to slightly different answers re the possible headway with 3-aspects; can you see why?
I think I am right- but tell me of you think otherwise. However getting the right answer isn't what's actually important- it is about demonstrating understanding. In the time pressure of the exam then you won't have time to do it as well as you did. So yes you will need to practice to get your marks-per-minute up, but I am definitely getting the impression that "the penny has dropped".
b) I am NOT suggesting that you do this in the exam, but just to have a look at it to attempt to deepen your understanding and "get an intuitive feel".
I have drawn the signal positions for 4 aspects and assumed that if signalled for 3 aspects these positions would coincide buit that alternate ones would be "missing". I have had to use red and blue pens to depict the different aspect sequence on the same aspect sequence lines which at least allows ready comaprison. There wasn't the space to draw much of the second train's path in (and ideed had I drawn in both the 3-aspect and 4-aspect case it would have become confusing, but the first portion is really the bit we need and it does show the difference in closeness reasonably well).
It is up to you as you'll know better than me how happy you feel, but I think that for now you have probably done enough on headway and ought to be moving on shortly to another exam element.
So why not now start placing your signals on the layout based on the results of your headway calcs and see how it starts working out in practice?
PJW