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COMPROMISED OVERLAP
#1
Can anyone explain me the term 'COMPROMISED OVERLAP'?
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#2
(25-10-2012, 09:04 AM)vikrantsharma06 Wrote: Can anyone explain me the term 'COMPROMISED OVERLAP'?

I am not familiar with the actual term you use there, not something I am aware of in UK practice. We refer commonly to Restricted Overlap (where a full overlap is not always available and an alternative, shorter overlap is available but with the mitigation that the signal in rear of the one held at danger is approach released from red) or a reduced overlap (where the layout is such that a standard overlap is not possible and, in sympathy with the speed of approach and assessment of other risk factors, a shorter overlap is tolerated).

The term compromised sounds more like something where an overlap has become unavailable because of something having entered it.

Peter
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#3
(25-10-2012, 09:04 AM)vikrantsharma06 Wrote: Can anyone explain me the term 'COMPROMISED OVERLAP'?

Actually it is a London Underground term.
Whereas the overlap on Network Rail has traditionally been a rather nominal length, because LU has had train protection in the form of train stops for >100 years, the LU overlap is a calculated distance.
The actual formula is complex and I am approximating, but basically it is long enough to contain a train which SPADs at maximum line speed and then suffers an intervention and hence is brought to a halt by emergency (i.e. maximum rate) braking.

It is not always possible though to give an overlap of the length needed without being too restrictive as too much of the layout is locked up and other train movements cannot be made.
Therefore a shorter overlap is provided which is referred to as a compromised overlap (though haven't seen that term written down in a standard); the mitigation for this is that a train is prevented from approaching the signal at a higher speed than for which that overlap is sufficient to contain the train should it SPAD. This is achieved by provision of a closing up signal with a trainstop or possibly blind trainstops (i.e. trainstops not associated with a lineside color light signal); there are special timing relays for track occupation and the circuits are arranged such that the trainstops fall only just before a correctly driven train reaches them. Hence if the train operator (driver) isn't braking soon enough, hard enough, then the train will get to a trainstop position too soon. Therefore it will still be up, the tripcock on the train will be physically hit and this opens the brake pipe which initiates emergency braking; this ensures the train stops safely within the compromised overlap. Sometimes there has to be a whole sequence of trainstops and they can be observed (and their air operation heard) from platforms such as Edgware Road Circle line as trains run in.
So in a way it is a bit like NR's Restricted overlap described by Peter; definitely not the same- there is no associated warning route etc,. but there is a certain similarity.

Now that NR has TPWS as part of a scheme design (rather than the situation of TPWS provided as a retrofit addition to existing signalling), the current standard requires the level of TPWS OSS loop provision to be determined such that the considered trains will be stopped within the overlap length- hence there has been a bit of a move towards the LU methodology. When NR shortens an overlap however this is "justified by risk assessment" rather than actually requiring it to be validated against the stopping distance given the TPWS provision, although clearly this is a relevant factor. There is a difficulty in that the published minimum lengths of restricted overlaps for various speeds and gradients pre-dates TPWS and to be honest really isn't compatible with the protection TPWS can give. I may be out of date ion this one but I do not think that NR requires the train protection to be able to stop a train within the restricted overlap.

This is one manifestation of LU regarding rear end collisions as much more significant than NR. There are a number of good reasons for this mainly due to the risk profile. An average slow speed rear end collision on NR will probably not have very major consequences because of the nature off the rolling stock and environment. On the Underground the density of traffic raises the risk in terms of likelihood of the SPAD, the likelihood of there being another train just ahead, the consequence of an accident is much greater due to the lesser crash resistance of the rolling stock, the greater number of people per carriage many of whom are standing and most crucially the fact that much of the network is in tunnel that is only just greater diameter than the train- so there is nowhere for either train to be deflected towards relatively safely but instead crushing is basically inevitable.


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PJW
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#4
(25-10-2012, 07:43 PM)Peter Wrote:
(25-10-2012, 09:04 AM)vikrantsharma06 Wrote: Can anyone explain me the term 'COMPROMISED OVERLAP'?

I am not familiar with the actual term you use there, not something I am aware of in UK practice. We refer commonly to Restricted Overlap (where a full overlap is not always available and an alternative, shorter overlap is available but with the mitigation that the signal in rear of the one held at danger is approach released from red) or a reduced overlap (where the layout is such that a standard overlap is not possible and, in sympathy with the speed of approach and assessment of other risk factors, a shorter overlap is tolerated).

The term compromised sounds more like something where an overlap has become unavailable because of something having entered it.

Peter

thanks Peter, as PJW mentioned below it is a LU term even my seniors did not aware of this in India.

(25-10-2012, 09:48 PM)PJW Wrote:
(25-10-2012, 09:04 AM)vikrantsharma06 Wrote: Can anyone explain me the term 'COMPROMISED OVERLAP'?

Actually it is a London Underground term.
Whereas the overlap on Network Rail has traditionally been a rather nominal length, because LU has had train protection in the form of train stops for >100 years, the LU overlap is a calculated distance.
The actual formula is complex and I am approximating, but basically it is long enough to contain a train which SPADs at maximum line speed and then suffers an intervention and hence is brought to a halt by emergency (i.e. maximum rate) braking.

It is not always possible though to give an overlap of the length needed without being too restrictive as too much of the layout is locked up and other train movements cannot be made.
Therefore a shorter overlap is provided which is referred to as a compromised overlap (though haven't seen that term written down in a standard); the mitigation for this is that a train is prevented from approaching the signal at a higher speed than for which that overlap is sufficient to contain the train should it SPAD. This is achieved by provision of a closing up signal with a trainstop or possibly blind trainstops (i.e. trainstops not associated with a lineside color light signal); there are special timing relays for track occupation and the circuits are arranged such that the trainstops fall only just before a correctly driven train reaches them. Hence if the train operator (driver) isn't braking soon enough, hard enough, then the train will get to a trainstop position too soon. Therefore it will still be up, the tripcock on the train will be physically hit and this opens the brake pipe which initiates emergency braking; this ensures the train stops safely within the compromised overlap. Sometimes there has to be a whole sequence of trainstops and they can be observed (and their air operation heard) from platforms such as Edgware Road Circle line as trains run in.
So in a way it is a bit like NR's Restricted overlap described by Peter; definitely not the same- there is no associated warning route etc,. but there is a certain similarity.

Now that NR has TPWS as part of a scheme design (rather than the situation of TPWS provided as a retrofit addition to existing signalling), the current standard requires the level of TPWS OSS loop provision to be determined such that the considered trains will be stopped within the overlap length- hence there has been a bit of a move towards the LU methodology. When NR shortens an overlap however this is "justified by risk assessment" rather than actually requiring it to be validated against the stopping distance given the TPWS provision, although clearly this is a relevant factor. There is a difficulty in that the published minimum lengths of restricted overlaps for various speeds and gradients pre-dates TPWS and to be honest really isn't compatible with the protection TPWS can give. I may be out of date ion this one but I do not think that NR requires the train protection to be able to stop a train within the restricted overlap.

This is one manifestation of LU regarding rear end collisions as much more significant than NR. There are a number of good reasons for this mainly due to the risk profile. An average slow speed rear end collision on NR will probably not have very major consequences because of the nature off the rolling stock and environment. On the Underground the density of traffic raises the risk in terms of likelihood of the SPAD, the likelihood of there being another train just ahead, the consequence of an accident is much greater due to the lesser crash resistance of the rolling stock, the greater number of people per carriage many of whom are standing and most crucially the fact that much of the network is in tunnel that is only just greater diameter than the train- so there is nowhere for either train to be deflected towards relatively safely but instead crushing is basically inevitable.

thanks very much PJW,it is very useful to understand me some documents.
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#5
The terms emergency and maximum, with respect to braking rates, should not be interchanged freely.

Effects of a collision in a confined space are detailled in the Moorgate accident report. Although the train hit a static object, the deformation and destruction would be akin to a collision between two trains.
Le coureur
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