2009 Main Line Layout

[Hort]

hi,
When carrying out headway calcs for a non-stopping train following a stopper, could someone explain how the additional time equates to the Textra = Tdwell + 0.5(Braking time + acceleration time)?

How is the 0.5(Braking time + acceleration time) portion derived? I had a go at deriving it using Newton's laws and substitiution but can't get my head around it!

[reuben]

Hi

You can prove it using the equations of motion - the key trick being to eliminate the distance taken to accelerate/ decelerate, which you don't know or want to know!

It's easier to do it in words:

Under constant acceleration, the mean velocity during a period of accn from 0 to V or decn from V to 0 will be V/2.

Therefore, it will take exactly twice as long to cover the same distance, during braking or acceleration, compared to going at full speed.

from equations of motion, time to accelerate = v/a, hence extra time taken to accelerate = v/2a.

This effect is repeated for both the deceleration and acceleration phases, hence Textra = 0.5 * (decn time + accn time) + Tdwell

You can also demonstrate this using a velocity-time graph, the area under this graph is the distance travelled. Comparing graphs for constant speed and station stopping, the difference in area is

V * (0.5 * (decn time + accn time) + Tdwell)

and hence 0.5 * (decn time + accn time) + Tdwell is extra time taken to cover the same distance.

hope this helps

Reuben

[Hort]

Thanks for your help

[Robbie]

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]

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.

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

[Robbie]

Hi PJW,

Thanks for your very helpful and prompt reply. I can see that what I wrote re: freight trains effectively treating a double yellow as a green is indeed politically incorrect nowadays. Very true

Just to confirm though, at the start of this thread, Reuben's suggested solution stated in the assumptions:
Maximum permitted distance from caution to red = 1.5 * minimum figure

If 3 aspect signalling is used for mixed traffic in the 2009 module 2 case, the distance from caution to red would be 2x the minimum figure for freight trains. Is this OK?

I forget exactly what the old O.S. Nock green IRSE textbook said about having a maximum of 1.5 times the braking distance. Is the main issue that the headway would be impacted if the 1.5 factor is exceeded (mind you, freight headway isn't an issue on this exam)? Or is there a safety issue eg. drivers running on a yellow aspect may "forget" (!) they are approaching a red, as the red is so far from the yellow? Or to say it another way, does having an excessive distance between a caution and a red breed train handling complacency?

I guess in summary, can you confirm Reuben's assumption is valid? If so, is it a safety issue and/or a headway issue or something else?


Also, if you don't mind some very basic questions while I'm at it:

a) I note that the exam question says to define all routes, however, Reuben's solution only includes route boxes for junction signals. Is this because not all routes need defining, or because of a UK practice I'm not aware of, or because the other routes are simple and so have been omitted to save time?

b) Also, please confirm, we don't need to show train protection for Main Line layouts, assuming the question doesn't ask for this. I only ask because Reuben's drawing shows some effort applied to include train protection, plus you mention previously a general AWS/TPWS note should be applied (as well as a specific note about the buffer stop and fixed red). Will this attract any marks, since the question doesn't explicitly ask for this?

Thanks so much,
Robbie

[Peter]

Hi PJW,

Thanks for your very helpful and prompt reply. I can see that what I wrote re: freight trains effectively treating a double yellow as a green is indeed politically incorrect nowadays. Very true

Just to confirm though, at the start of this thread, Reuben's suggested solution stated in the assumptions:
Maximum permitted distance from caution to red = 1.5 * minimum figure

If 3 aspect signalling is used for mixed traffic in the 2009 module 2 case, the distance from caution to red would be 2x the minimum figure for freight trains. Is this OK?

I forget exactly what the old O.S. Nock green IRSE textbook said about having a maximum of 1.5 times the braking distance. Is the main issue that the headway would be impacted if the 1.5 factor is exceeded (mind you, freight headway isn't an issue on this exam)? Or is there a safety issue eg. drivers running on a yellow aspect may "forget" (!) they are approaching a red, as the red is so far from the yellow? Or to say it another way, does having an excessive distance between a caution and a red breed train handling complacency?

The 1.5 figure is UK practice before a specific risk assessment is required. This Railway Group Standard sets out the conditions - see section 2.

I guess in summary, can you confirm Reuben's assumption is valid? If so, is it a safety issue and/or a headway issue or something else?

It may cause a headway issue, but the safety element would be more important.

Also, if you don't mind some very basic questions while I'm at it:

a) I note that the exam question says to define all routes, however, Reuben's solution only includes route boxes for junction signals. Is this because not all routes need defining, or because of a UK practice I'm not aware of, or because the other routes are simple and so have been omitted to save time?

I would not expect to see a route table for a signal with only one route unless it has something odd about it (eg an approach release condition for deficient braking.

b) Also, please confirm, we don't need to show train protection for Main Line layouts, assuming the question doesn't ask for this. I only ask because Reuben's drawing shows some effort applied to include train protection, plus you mention previously a general AWS/TPWS note should be applied (as well as a specific note about the buffer stop and fixed red). Will this attract any marks, since the question doesn't explicitly ask for this?

It is quite normal to put a blanket comment about standard AWS provision and that no other train protection is shown.

Thanks so much,
Robbie

Peter (not PJW)

[PJW]

If 3 aspect signalling is used for mixed traffic in the 2009 module 2 case, the distance from caution to red would be 2x the minimum figure for freight trains. Is this OK?
.....
Or to say it another way, does having an excessive distance between a caution and a red breed train handling complacency?

I guess in summary, can you confirm Reuben's assumption is valid? If so, is it a safety issue and/or a headway issue or something else?

To add to Peter's statement given above, there is inevitably overbraking for any trains running (because of their maximum permitted speed, their timetable, their attainable having joined a line from another of lower speed / after a station stop etc) at less than the speed at which the line is signalled (excepting where the reason why those trains are to run slower is due to their less good braking). We can't do anything about this and anyway we claim that driver's comprehend. What we are seeking to achieve is the complacency that can arise if they habitually find that vastly excessive distances exist between the caution signal and the red, so that they get accustomed to slowing only is step 1 braking for example; this leads to a trap when there is a signal section that really does need step 3 braking because of its short length. Actually there is an argument that CONSISTENCY along a route is just as important as the actual degree of overbraking.

I note that the exam question says to define all routes, however, Reuben's solution only includes route boxes for junction signals. Is this because not all routes need defining, or because of a UK practice I'm not aware of, or because the other routes are simple and so have been omitted to save time?

In most cases it should be immediately obvious to anyone where a signal with one route reads to; however where there is any ambiguity then there is a need to define- that could be where the track layout would actually permit more routes than the signalling system makes use of.
Where there is a junction then a route box is neede to define what, if any, approach release is envisaged, what route indicator is to be shown etc. As Peter says there are also instances when there is something "special"- perhaps it could be needing a slot from a shunter into a depot etc- and a relevant note is needed, so a route box can be a good way of showing.
In reality you are unlikely to have the time to do all that ideally you should- concentrate on the most important, which may mean only doing one of a group of parallel related signals so that you can devote your time elsewhere to give the best overall coverage. Don't waste time on simple ones that add little.

Also, please confirm, we don't need to show train protection for Main Line layouts, assuming the question doesn't ask for this. I only ask because Reuben's drawing shows some effort applied to include train protection, plus you mention previously a general AWS/TPWS note should be applied (as well as a specific note about the buffer stop and fixed red). Will this attract any marks, since the question doesn't explicitly ask for this?

Again it is a question of where to spend your time for maximum benefit. Certainly I wouldn't show the detail which Reuben has for the IRSE exam. However don't ignore train protection completely, so coverage by a detailed note s what I'd recommend, plus perhaps showing a typical associated with that note or there is a particular relevance (perhaps on a barely signalled single line branch).

Look at the module 3 layouts provided for Control tables as a good guide to what the examiners expect re detail on a module 2 layout.

Actually I think that the low prioritisation of train protections is one area in which the IRSE Exam has not kept pace with the UK rail industry- it basically reflects the pre-1990s view, which is significantly at variance to current practice in which we justify the overlaps provided partly on the basis of the effectiveness of train protection, so leaves me uncomfortable not to include at all. I guess that is why Reuben when producing a model layout to learn from (rather than on for production in the exam) shows as much as he does. Time constraints exist in the exam though and hence that is why I feel that stating a detailed assumption is the best way to address.

This comment applies to MAINLINE layouts; for METRO train protection is far more important (actually arguably more important as signals than the lights on sticks, where provided).

[reuben]

I must confess that I have a difference of opinion here over the provision of AWS and TPWS on your layout.

The question says "signal" the layout...... - does this mean that there are no marks for shunt signals, limits of shunts, route indicators, and indeed for showing that a lightly -used line would use a staff instead of physical signals?

Equally, you can't say "I will provide ERTMS level 2 which uses no signals, please can I have 60 marks?"

We are railway signal engineers in the widest sense, and I believe that the verb "to signal", which is the instruction in the question, is to provide appropriate signalling in the widest sense of the word.

Putting it another way, signalling causes trackside devices (signals, AWS, TPWS, eurobalises, coded track circuits, GSM-R base-stations) to transmit various types of radiation, which is received by trainborne receivers for train advice, train protection, ATO etc. Why should the question be limited to the devices which emit only in the visible spectrum?

I strongly believe that AWS and TPWS are an integral part of the current UK main line signalling philospohy, just as much a part as eurobalises in an ERTMS system or coded track circuits which transmit a speed code in other systems. Therefore, showing the examiner that you understand these integral parts, and how to correctly apply them, is an essential "signalling" skill.

I don't hold with the view of writing a general comment "all main signals fitted with AWS...." - the question asks you to do it, not write an essay about it!, and crucially, you miss the opportunity to show that you can correctly identify the signals which do not have AWS, and those which do not have TPWS.

Finally, you'll never have time to create a complete layout in the exam. In this context, i strongly beleive in showing that you can do " a little bit of everything" : main signals, route indicators, controlled signals, automatic signals, shunt signals, limits of shunt, local manouevres etc. This shows much more breadth of knowledge compared to just having time to do all the main signals and running out of time. Bolting- on AWS and TPWS when appropriate adds to this picture of breadth of knowledge which you can use to impress the examiner.

Reuben

[Robbie]

What an excellent forum this is! I have received three replies from three moderators, each clearly with a wealth of experience. Thanks so much to you all for your help.

Sorry to be a pain on the 1.5 times the braking issue. I may be slow here, but from my reading of the GK/RT0075 standard, I can't seem to see where it says it's OK to exceed the min braking distance by more than 50% for mixed traffic on the same line. However, as pointed out, I guess this is one of those problems that arises in the exam because the deceleration rates for passenger and freight are equal, and this is not necessarily reflective of reality (though makes for simpler exam calculations).

Point taken about train protection. I'll see how I go with it on the exam. Will definitely put a comment on at the very least, and hopefully more, time permitting.

Thanks again,
Robbie