Stopping Calcs - Turn Outs

[dfreer]

Does anyone know how to deal with turn outs when calculating stopping headways? If you have a brach off a main line (100km/h to 80km/h) via a 40km/h turn out do you have to include the 40km/h turn out in your calcs and the associated acceleartion / deceleration involved or do you just take the 'simple case' of the branch line speed and take it from there?

I'm looking at the 2002 examination layout 1 and (hopefully) am making things needlessly complicated.

Any help gratefully received.

[PJW]

Does anyone know how to deal with turn outs when calculating stopping headways? If you have a branch off a main line (100km/h to 80km/h) via a 40km/h turn out do you have to include the 40km/h turn out in your calcs and the associated acceleartion / deceleration involved or do you just take the 'simple case' of the branch line speed and take it from there?

I'm looking at the 2002 examination layout 1 and (hopefully) am making things needlessly complicated.

Any help gratefully received.

To calculate headway accurately does require the calculations to reflect the speed profile that the train really adopts. Hence it should be obvious that if a train has to respect a speed restiction anywhere (and this includes the length of a turnout) that this should be factored into the sums and of course the decelration to that speed and reacceleration afterwards.

Indeed if the branch speed is less that that of the mainline (by greater than 10mph) then the junction signal would almost certainly be approach released (assuming UK mainline 3 or 4 aspect MAS practice). Hence the train speed on the approach to the junction would be constrained by the driver seeing restrictive aspects and your calculations would need to model that.

So in principle the answer is yes, absolutely definitely. However are you sure that the stated headway requirement applies to the branch?
In the example quoted the branch is only a single track line with no passenger passing loop but only an intermediate freight facility so the speed over turnouts is hardly the determining factor for "headway" is it? STOP TO THINK, don't just tap in numbers into a calculator determined by a formula.


READ THE NOTES: Contracted paths A to F, one passenger train per hour by day and by night 2 freight trains to the intermediate freigh facility.

It even says Required Headway for Mainlines.......

regards,
PJW

[dfreer]

Thanks PJW, much appreciated.

I think my layout might be slightly different to yours as my destination 'F' is a level crossing but I take your point.

Does anyone know how to deal with turn outs when calculating stopping headways? If you have a branch off a main line (100km/h to 80km/h) via a 40km/h turn out do you have to include the 40km/h turn out in your calcs and the associated acceleartion / deceleration involved or do you just take the 'simple case' of the branch line speed and take it from there?

I'm looking at the 2002 examination layout 1 and (hopefully) am making things needlessly complicated.

Any help gratefully received.

To calculate headway accurately does require the calculations to reflect the speed profile that the train really adopts. Hence it should be obvious that if a train has to respect a speed restiction anywhere (and this includes the length of a turnout) that this should be factored into the sums and of course the decelration to that speed and reacceleration afterwards.

Indeed if the branch speed is less that that of the mainline (by greater than 10mph) then the junction signal would almost certainly be approach released (assuming UK mainline 3 or 4 aspect MAS practice). Hence the train speed on the approach to the junction would be constrained by the driver seeing restrictive aspects and your calculations would need to model that.

So in principle the answer is yes, absolutely definitely. However are you sure that the stated headway requirement applies to the branch?
In the example quoted the branch is only a single track line with no passenger passing loop but only an intermediate freight facility so the speed over turnouts is hardly the determining factor for "headway" is it? STOP TO THINK, don't just tap in numbers into a calculator determined by a formula.


READ THE NOTES: Contracted paths A to F, one passenger train per hour by day and by night 2 freight trains to the intermediate freigh facility.

It even says Required Headway for Mainlines.......

regards,
PJW

[PJW]

Thanks PJW, much appreciated.

I think my layout might be slightly different to yours as my destination 'F' is a level crossing but I take your point.

Oops sorry! I took the adjacent (2001) layout out of my folder in error!

In reality for the 2002 layout I suggest the calculation should initially assume that the train entering the branch keeps at 40km/h until braking to stop in station . If that worst case achieves the headway then solution is obviously OK, otherwise then calculate how much time could be saved if train accelerated for a short length (couldn't accelerate until last vehicle had passed over the branch points) prior to decelerating again to a stop in station.

[NB. Assuming the junction signal had only displayed yellow + PLJI, we wouldn't really want to encourage the driver to accelerate after the speed restriction, seeing that would be approaching a red]

For the opposite direction perhaps it is more reasonable to expect the driver to accelerate to more than 40km/h and then slow again to take the junction.

The more general issue is that it can be sensible to approximate to make the calculations easier, but do ensure that you do so in a way that addresses the worst case. Round UP braking distances, round DOWN the non-stop headway distance, assume the lower speed for headway calcs where speed varies etc.

PJW
PJW

[nthomso3]

Peter,

Would this mean a calculation is need for the initial section of line going into the first station, then a seperate calculation for after the first station to account for the greater approach speed?

Neil

[PJW]

Peter,

Would this mean a calculation is need for the initial section of line going into the first station, then a separate calculation for after the first station to account for the greater approach speed?

Neil

If you really have to assess the headway for a line then yes you have to do the lot to ensure that the required headway can be delivered. However remember that headway is determined by the WORST section. No way does it "average"; it is limited by the bottle neck. You don't need to calculate the headway that would be achieved by each particular short stretch of signaling in isolation; just that overall your solution delivers. Thus you need to spot what is likely to be the limiting section and prove that is OK.

Hence in this case I suggest you concentrate on the area around station D and if you have the same signal spacing relative to station G then you know that will be better. The issue is that other constraints (e.g. that level cossing- signal on the approach and also constraint re standage re the position that the one beyond it can sensibly be placed) may force you to increase signal spacing so may not be intuitive obvious that it will achieve. However in the exam you'll be too pushed for time to redo calcs- the stopping calcs are only about 10% marks so just can't afford to take any more than 9 minutes. Once you've shown you can do calcs it'll certainly be the law of diminishing returns- you are better investing your time elsewhere ........

[PJW]

Note that there is some pertinent information / discussion re calculations of stopping headway in slightly different context in Module 3 thread