22-09-2012, 08:53 AM
There are definite advantages in 4 aspects when there is mixed traffic. However whereas what you wrote re the trains with shorter braking distances effectively treating double yellows as greens isn't true nowadays. In the past it certainly was; now with defensive driving to the fore then it is most politically incorrect to suggest. The IRSE textbooks of the 1950s-1980s are therefore misleading in the modern context.
Actually some of the problem in the exam papers arise because the IRSE always seem to set the braking rates for the freight to be the same as the passenger for simplicity. The reality is generally that freight operate at lower speed than the passenger BECAUSE they have lower braking rate, although I accept that there are also civil / mechanical engineering reasons why lower speeds due to axle loads, vehicle dynamics etc are needed.
One of the reasons why 4 aspects are advantageous where disparity of speeds is that 3 aspects spaced for high line speed are necessarily a long way apart. Hence the block sections take a long time to clear when used by slower traffic or particularly in degraded mode where there is a severe speed restriction or need to procedurally authorise a train past a signal at danger.
For the exam, you need to convince examiner that your solution is
a) safe
b) operable
c) economical.
Since almost twice as many signals and also more train detection sections are needed for 4-aspects, then you have to justify their use. For areas of your layout where the demanded capacity does not directly dictate their use, you will need to explain by means of a note why you have made the appropriate choice- you are effectively explaining to your client why it is in their best interests to invest in both capital expenditure and ongoing maintenance and reliability costs more than they would have expected. So need to look at the particular case on its merits, rather than quote any rule of thumb; having said that if dealing with line speeds over 100mph this is one very significant factor- due to the length of block sections which result being operationally difficult in degraded mode as explained above.
Also consider that the examiners can tend to ee through excuses for avoiding complication of 3 to 4 aspect transitions, also that if you draw 4 aspects throughout then you are giving yourself more to do in the time allowance.
Actually some of the problem in the exam papers arise because the IRSE always seem to set the braking rates for the freight to be the same as the passenger for simplicity. The reality is generally that freight operate at lower speed than the passenger BECAUSE they have lower braking rate, although I accept that there are also civil / mechanical engineering reasons why lower speeds due to axle loads, vehicle dynamics etc are needed.
One of the reasons why 4 aspects are advantageous where disparity of speeds is that 3 aspects spaced for high line speed are necessarily a long way apart. Hence the block sections take a long time to clear when used by slower traffic or particularly in degraded mode where there is a severe speed restriction or need to procedurally authorise a train past a signal at danger.
For the exam, you need to convince examiner that your solution is
a) safe
b) operable
c) economical.
Since almost twice as many signals and also more train detection sections are needed for 4-aspects, then you have to justify their use. For areas of your layout where the demanded capacity does not directly dictate their use, you will need to explain by means of a note why you have made the appropriate choice- you are effectively explaining to your client why it is in their best interests to invest in both capital expenditure and ongoing maintenance and reliability costs more than they would have expected. So need to look at the particular case on its merits, rather than quote any rule of thumb; having said that if dealing with line speeds over 100mph this is one very significant factor- due to the length of block sections which result being operationally difficult in degraded mode as explained above.
Also consider that the examiners can tend to ee through excuses for avoiding complication of 3 to 4 aspect transitions, also that if you draw 4 aspects throughout then you are giving yourself more to do in the time allowance.
(22-09-2012, 06:47 AM)Robbie Wrote: Hello,
Regarding the 2009 mod 2 layout, and similar layouts with mixed traffic - my approach was to use 4 aspect signalling purely because of the difference in braking distances of the traffic involved. The braking distance of one train type (say passenger) is twice that of the other train type (say freight). By using 4 aspect signalling, the passenger trains can start braking at the double yellow aspects while the freight trains can start braking at the single yellow aspects. Compared to a 3 aspect solution, the 4 aspect solution ensures a reasonable distance between the relevant yellow aspect and the red aspects for all traffic.
Having said this, I'd like to know whether this is a reasonable approach, or is this too simplistic?
Taking this further, whenever mixed traffic is present and the braking distances between passenger and freight differ by more than a factor of 1.5, would it be justifiable to use 4 aspect signalling (or should this be judged on a case by case basis)?
Many thanks,
Robbie
PJW