Overlap

[Vinoth Rajamani]

Hi,
 
Could you please throw some light on the following questions on Overlap,
 
i)             How overlap distance 180m is decided? Is it based on the historical data?
 
ii)            As per Group standard GK/RT0064 suggest, where it is not reasonably practicable to provide a full overlap, it is permissible, subject to risk assessment the overlap length to be less than as shown below,
 

Maximum Permissible/Attainable Speed not Exceeding - Minimum Overlap Distance

15 mile/h - 45m

20 mile/h - 55m

25 mile/h - 60m

30 mile/h - 70m

35 mile/h - 75m

40 mile/h - 80m

45 mile/h - 90m

50 mile/h - 105m

55 mile/h - 125m

60 mile/h - 135m

How the minimum overlap distance is derived as shown above?
iii)           As it is possible to have overlap greater than 180m distance, Does only operational flexibility decides this?
 
Regards,
Vinoth

[dorothy.pipet]

Hi,
There is no scientific basis on which this is calculated, for UK mainline. It was historically 200 yards for colour light signals = 183m in metric and became rounded to 180m. It works so we stick with it.

In some areas the standard overlap length has been increased to 225m - I think this is where the higher speed pendolino trains run (West Coast) but I'm sure somebody on here (PJW?) can confirm/deny that. In any case it is always possible to have a longer overlap, perhaps because 180m lies in pointwork, or there is already a need for an track joint (or axle counter) close to 180m but too close to allow another joint, or just for reasons of economy. In the Paddington area the overlaps are regularly around 200m, and not uncommon to find up to 400m.

Occasionally a phantom overlap is used where the overlap is nominally part-way through a track. In effect this is used where the overlap tests a track clear, but not certain points within that track where they are outside the Phantom overlap length.

[PJW]

Hi,
There is no scientific basis on which this is calculated, for UK mainline. It was historically 200 yards for colour light signals = 183m in metric and became rounded to 180m. It works so we stick with it.

In some areas the standard overlap length has been increased to 225m - I think this is where the higher speed pendolino trains run (West Coast) but I'm sure somebody on here (PJW?) can confirm/deny that. In any case it is always possible to have a longer overlap, perhaps because 180m lies in pointwork, or there is already a need for an track joint (or axle counter) close to 180m but too close to allow another joint, or just for reasons of economy. In the Paddington area the overlaps are regularly around 200m, and not uncommon to find up to 400m.

Occasionally a phantom overlap is used where the overlap is nominally part-way through a track. In effect this is used where the overlap tests a track clear, but not certain points within that track where they are outside the Phantom overlap length.

Yes longer overlaps came in at about the time of West Coast Route Modernisation when there was particular concern re SPADs and the rise of such things as Robust Train Protection.  Historically NR generally had no form of train protection so all overlaps rather nominal allowances; since circa 2000 the existence of TPWS has meant that there can be some form of more objective assessment where a train might stop following a SPAD, but it is nothing like to the detail and precision as on London Underground or indeed a more sophisticated form of train protection.  Obviously the longer the overlap the better the chance that a train will come to a safe stop within that length and by choosing a 225m overlap it means that the quantity of TPWS OSS loops to give the same level of confidence can be reduced; on a high speed line the extra length doesn't have much of a headway impact.

Conversely if you do the sums for the various lengths of reduced overlaps according to permissible speed, you actually find that the historically accepted vales are actually TOO SHORT for TPWS to be effective; therefore if starting afresh than one would probably come up with a completely different set of values.  They are what they are.
Remember these are all MINIMUM values; should endeavour to give as long an overlap as possible that is operationally acceptable.

[PJW]

Hi,
There is no scientific basis on which this is calculated, for UK mainline. It was historically 200 yards for colour light signals = 183m in metric and became rounded to 180m. It works so we stick with it.

In some areas the standard overlap length has been increased to 225m - I think this is where the higher speed pendolino trains run (West Coast) but I'm sure somebody on here (PJW?) can confirm/deny that. In any case it is always possible to have a longer overlap, perhaps because 180m lies in pointwork, or there is already a need for an track joint (or axle counter) close to 180m but too close to allow another joint, or just for reasons of economy. In the Paddington area the overlaps are regularly around 200m, and not uncommon to find up to 400m.

Occasionally a phantom overlap is used where the overlap is nominally part-way through a track. In effect this is used where the overlap tests a track clear, but not certain points within that track where they are outside the Phantom overlap length.

Yes longer overlaps came in at about the time of West Coast Route Modernisation when there was particular concern re SPADs and the rise of such things as Robust Train Protection.  Historically NR generally had no form of train protection so all overlaps rather nominal allowances; since circa 2000 the existence of TPWS has meant that there can be some form of more objective assessment where a train might stop following a SPAD, but it is nothing like to the detail and precision as on London Underground or indeed a more sophisticated form of train protection.  Obviously the longer the overlap the better the chance that a train will come to a safe stop within that length and by choosing a 225m overlap it means that the quantity of TPWS OSS loops to give the same level of confidence can be reduced; on a high speed line the extra length doesn't have much of a headway impact.

Conversely if you do the sums for the various lengths of reduced overlaps according to permissible speed, you actually find that the historically accepted vales are actually TOO SHORT for TPWS to be effective; therefore if starting afresh than one would probably come up with a completely different set of values.  They are what they are.
Remember these are all MINIMUM values; should endeavour to give as long an overlap as possible that is operationally acceptable.

From my analysis of SPADS at the time when TPWS was being rolled out, I can tell you that nearly 90% of the SPADs which I looked at where TPWS was not involved stopped within 180m of the signal; hence one could argue that it would have been a reasonable figure to have chosen on the basis that it reduced collision likelihood by about an order of magnitude and hence perhaps that gives it some retrospective justification.