2009 Q6 - Level Crossings & Cab signalling

[interesting_signal]

I'm struggling with part 2 (iii) of this question which asks to "Describe the potential impact on the strike in distance of a change to a cab signalling system". (This is for a crossing with four barriers which close the road completely, and barriers close automatically).

It's only worth 2 marks in the question so don't need to say too much, but I really don't know much on this topic. Is information such as strike in points easily updated in cab signalling systems?

Thanks.

[PJW]

I'm struggling with part 2 (iii) of this question which asks to "Describe the potential impact on the strike in distance of a change to a cab signalling system". (This is for a crossing with four barriers which close the road completely, and barriers close automatically).

It's only worth 2 marks in the question so don't need to say too much, but I really don't know much on this topic. Is information such as strike in points easily updated in cab signalling systems?

Thanks.

It is a it of an odd question. I think your answer could include:

1. Assumption whether there is lineside train detection. If not then the level crossing would have to be triggered by the position reports from the trains themselves and therefore how frequently they are polled, the likelihood of communication drop out and the degree of certainty with which they know their position could all factor. Certainly wouldn't be comfortable with operating an AHBC like this but in the case of CCTV if the LX not initiated soon enough then the train will get delayed rather than be unsafe.

2. Similarly the time delay between the level crossing having achieved down detection with crossing clear,via the interlocking, the RBC, the EVC (assuming ETCS) before the MA extension is visible to the driver and the ATP is relevant.

So I think it is more to do with unintentional system latency than anything else. Don't forget, if you want something implemented quickly, use relays and the speed of light between the signal and the driver's eye rather than many separate processors that may be changing 0s and 1s at the rate of GHz yet takes an age to do anything macroscopic!

[Jerry1237]

It would also depend if it was continuous ICS or intermittent. If it were intermittent, there would be little, if any change, to the strike in distance bearing PJW's point about letency in mind (processors always take longer than relays!). With continuous, you could potentially argue a variable strike-in based upon the trains' braking performance which would decrease the strike in time which obbviously has an impact on distance. A presumption is that strike-in could be as little as braking distance plus a safety margin at line speed. That should obviously not increase the BD as it [the strike in] should already be at braking distance plus margin of the worst performing train!

There is an arguement that train bourne signalling with geographic data plus redundant balises are not quite as laggy as CBIs of old but I have no figures to support that assertion.

Jerry

[PJW]

Yes that is a good point which hadn't occurred to me; a slow moving freight would not need to call for the CCTV level crossing to be lowered anything like as soon as an express. A bit like a level crossing predictor for AHBC; there would not be a "strike-in" point as such.
The train on-board would be analysing where it would need to start braking to stop prior to the crossing, adding an allowance for the time between asking for the barriers to lower and obtaining its extension of MA (sum of system response times including the somewhat unpredictable human giving crossing clear- unless replaced by radar detection) and then calculating back at its approach speed the place at which it would need to make that request in order to get its MA extension in time.
That must be worth the 2 marks!

It would also depend if it was continuous ICS or intermittent. If it were intermittent, there would be little, if any change, to the strike in distance bearing PJW's point about letency in mind (processors always take longer than relays!). With continuous, you could potentially argue a variable strike-in based upon the trains' braking performance which would decrease the strike in time which obbviously has an impact on distance. A presumption is that strike-in could be as little as braking distance plus a safety margin at line speed. That should obviously not increase the BD as it [the strike in] should already be at braking distance plus margin of the worst performing train!

There is an arguement that train bourne signalling with geographic data plus redundant balises are not quite as laggy as CBIs of old but I have no figures to support that assertion.

Jerry

[Jerry1237]

Mine is a systems engineers' answer to a signalling question!

[ricky]

Might be a little late know with the exam on Saturday but wondered if i could get a little bit more feedback on the question as a whole.

The question asks to derive a strike in running at 140km/h with three aspect signalling with minimal delays to the rail traffic.

I approached this by drawing a diagram which showed the crossing the 3 signals in approach and the overlap of the final signal before the crossing. I assume that for minimal delays a train driver would only encounter green aspects.

I am a little unsure of braking distances and signal sighting distances so I assumed 800m to stop from 140km/h and 200m to see a signal. This meant that the two signals closest to the crossing were 800m apart. This plus the 180m overlap plus an extra 10m as the overlap wouldn't want to be in the crossing area means the final signal must display a green aspect when the train is 1190m from the crossing.

The crossing sequence should show amber for 3 secs before red lights for 6s before the entrance barriers lower. Then 8s for the barriers to lower and the signaller to confirm the crossing was clear before the exit barriers lower which shall take a further 6seconds. This totals 23secs. 23secs at 140km/h is 894m so the strike-in should be 2084m from the crossing.

i) I don't think 4 aspect would affect this distance as the driver is only seeing green aspects but the three signals closest to the crossing will be 800m apart (2x400m) so the strike in is the same

ii) this may save a couple of seconds @ 140km/h is 78m closer to the crossing.

iii) the sighting distance will no longer be required so 200m closer to the crossing.

On the final part I was really unsure so if anyone has anything they can suggest on this I would appreciate it. The only thing I know about in this respect are level crossing predictor technology that uses the change in impedance to calculate the speed of the train and whether it is deccelarating. This could be used with a treadle for stopping trains but i am not sure of the traditional meathods.


Thanks for reading

[PJW]

I approached this by drawing a diagram which showed the crossing the 3 signals in approach and the overlap of the final signal before the crossing. I assume that for minimal delays a train driver would only encounter green aspects.

I am a little unsure of braking distances and signal sighting distances so I assumed 800m to stop from 140km/h and 200m to see a signal. This meant that the two signals closest to the crossing were 800m apart. This plus the 180m overlap plus an extra 10m as the overlap wouldn't want to be in the crossing area means the final signal must display a green aspect when the train is 1190m from the crossing.

Methodolgy seems sound but distances rather low. 800m sounds to be rather like braking of a 9percent g train from around 75mph and thus rather an understimate for 140km/h. Also sighting distance at that speed would probablly be more like 300-400m, but the distances are not rediculous and it is the principle that the question is after, so good enough for that.

The crossing sequence should show amber for 3 secs before red lights for 6s before the entrance barriers lower. Then 8s for the barriers to lower and the signaller to confirm the crossing was clear before the exit barriers lower which shall take a further 6seconds. This totals 23secs. 23secs at 140km/h is 894m so the strike-in should be 2084m from the crossing.

OK except that the signalman gives crossing clear after ALL the barriers are down; you probably need to allow at least another 5 seconds for their attention to be drawn, finish the task in which engaged (i.e. stop reading the newspaper- I meant their Rule Book!), look carefully at the CCTV monitor and then push and release the CC button.

i) I don't think 4 aspect would affect this distance as the driver is only seeing green aspects but the three signals closest to the crossing will be 800m apart (2x400m) so the strike in is the same

The crossing protecting signal starts at red and for 3 aspects the on which changes from Green is 2 signal spacings (each a minimum of braking distance) away.
For 4 aspect the one which changes from Green is 3 signal spacings (each a minimum of half braking distance) away.
Therefore not quite the same.

Also if get crossing down slightly too late so that driver gets cautionary aspect the train will be realy stopped by the time it gets to see the signal ahead is no longer at red; on 4 aspect signalling then the driver will get an update at the signal they had to expect to be single yellow but by then may well have cleared and thus can stp braking and reaccelerate befor so much speed had been lost.

ii) this may save a couple of seconds @ 140km/h is 78m closer to the crossing.

iii) the sighting distance will no longer be required so 200m closer to the crossing.

On the final part I was really unsure so if anyone has anything they can suggest on this I would appreciate it. The only thing I know about in this respect are level crossing predictor technology that uses the change in impedance to calculate the speed of the train and whether it is deccelarating. This could be used with a treadle for stopping trains but i am not sure of the traditional meathods.


Thanks for reading

[ricky]

Thanks Peter,

Glad to know i was along the right lines.