2011 Headway calculation

[PJW]

[quote='kball' pid='3438' dateline='1330550186']
Hello,
I have recently joined the IRSE forum with the intention to sit the IRSE exam this year.
It's been a while since I tried any headway/ braking distance calculations and I would be grateful if someone would be kind enough to look over my calcultions and provide some feedback.

Kind regards
KJB

I agree with Jerry that this is too long for examination timescales; you really need to get all the calculations done within 20-25 minutes and this means also the stopping headway ones which you have not really attempted.
Be aware that there should be differences between:
a) Textbook explaining to students
b) A student’s preparatory work produced as practice to help them absorb the information
c) An exam candidate’s response.

Answer (ignoring the repeated question) started well, but do not give braking distances allegedly to the centimetre! Round up to the next higher 5m or 10m; suitable level of precision.
Since freight train braking rate is given as the same as passenger, do not need to consider separately; worth stating the (rather unjustified) assumption of no significant brake build up time though! Not sure why felt need to calculate for what you called “trains” 4 & 5.
Headway is “required” (not “defined”) to be 2 minutes
Headway is not the distance between Green and Red; it is the time (which at constant speed can be converted to a distance) between the sighting point of the Green to the end of the overlap beyond the Red, (as the other respondents have already pointed out). Similarly as Zaphod has said, in 4 aspect signalling for signals at minimum spacing it is the double yellow which has to be braking distance from the red. Again when giving the calculated times then quote them to suitable precision.

Your answer showed lack of experience / comprehension by presenting 4 aspect signalling giving a worse headway than 3 aspects; getting a wrong answer is one thing, not recognising and acknowledging that it is ridiculous is what will lose you lots of marks.
Having calculated that 3 aspects just meet the constant 140km/h headway specification, you then state that such signalling is suitable; the fact that it is barely sufficient even with the signals at absolute minimum means that in practice you just couldn’t achieve a real scheme when there are other constraints re signal positioning. By adding on your arbitrary allowance for “unknown factors” you clearly will no longer be able to satisfy the headway requirement; you do not seem to have recognised that.

As others have pointed out, you can’t ignore the deceleration and acceleration times for the stopping trains; these are greater than the 30 seconds which you have added. Hence rather missed the whole point of this part of the question. The question actually asked for the situation where a non-stopping train was catching up an earlier stopping train.
So overall as an initial activity t start to get to grips with the subject then you have made a useful start. There are some useful components here, but it is not yet close to an exam answer.

Zaphod was right that you have some useful stuff to build upon but I feel he was being far too kind in an attempt to encourage you. Whereas this is a good thing, I think that I need to balance this by being rather more blunt.

You have shown you can read the question and put some numbers into equations, but you have not demonstrated signalling knowledge and understanding, rather the contrary. For example (in addition to what I have written prior) if 3 aspects barely deliver headway, then 2 aspects certainly wouldn’t (though of course might be just what is needed on some lighter used portions of the layout such as a branch). Note that 2 aspect signals are still generally used as a 3 aspect sequence and does not require Absolute Block Working.

So when reading your answer, I do not get the feeling of “competent signal engineer” but “someone who can do maths but doesn’t understand the signalling they are attempting to describe”. I might give you 5 of the 20 marks available for being able to calculate braking and have some idea about what non-stop headway is.

However it is early days and there is plenty of time to learn; by looking at other attempts on this website and looking at the comments on these as well as being prepared to try again and submit further attempts for more comments you’ll be able to improve.