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Reduced overlap
#11
Well actually there have been this type of shortened overlap for many many years- the only way they are identified is by a usual full overlap symbol with a measurement against it which is less than the old standard 200 yards (183 then 180m). A line on which I am working is the Wimbledon brnach of the District Line 9signalled from NR's Wimbledon ASC to normal standards of the time but with the addition of trainstops) which has overlaps generally about 90m. Similarly the branch line that terminates at Richmond as OL marked as 150 (yards).
Certainly from memory the whole of the North London Line in the vicinity of Camden Road (at least prior to the resignalling of about 5 years ago for London Overground) had overlaps of less than 100m and indeed the AWS was similarly positioned at a similar distance prior to the signals (compliant with he standards of the time and delivberately done due to low line speed).
Hence I think it is a matter of "can't see wood for trees"; such suburban lines often feature such overlaps, but perhaps the clue goes unobserved as it doesn't stick out unless you know what you are looking for!


(03-01-2013, 09:32 AM)jenni.joseph9 Wrote: Hi,

Thank you for the detailed explanation.

Is there a case where a reduced overlap is considered. I tried to search that, but could not find one.

Can you help me with that, please, if you have one..

Thanks & Regards,
PJW
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#12
(03-01-2013, 09:32 AM)jenni.joseph9 Wrote: Hi,

 Thank you for the detailed explanation.

Is there a case where a reduced overlap is considered. I tried to search that, but could not find one.

Can you help me with that, please, if you have one..

Thanks & Regards,

Yes, there are situations where a reduced overlap is considered to be appropriate and sufficient. This is where the approach speed to a signal having the reduced overlap is low and the provision of a full overlap would lead to unnecessary (usually) operational complexity.

On the UK main lines, for an example, if the permissible or attainable approach speed to a signal is 40 miles per hour, the overlap can be reduced to 80 metres as opposed to the standard 180 metres. For an approach/attainable speed of 60 miles per hour, the overlap can be reduced to 135 metres and for any approach speed greater than 60 miles per hour, a minimum of a standard overlap is required.

Sometimes reduced overlaps are also applied where train speeds are very low and the required throughput is high (e.g. on approach to major terminal stations where a high volume of trains terminate and turn around). Using standard overlaps in such cases would unnecessarily increase headways.
GS
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#13
If one goes back to the origins of overlaps when there was no train protection system then the length was to allow for misjudgement, driver microsleeps (although we did not call them that then), etc. History had led us in UK to a long distance being needed for a semaphore home signal (because at night it and the distant were lit by sleepy glowworms!) but for colourlight signals with cautionary aspects then a shorter distance could be used.
Where that distance was "inconvenient" then we allowed the distance to be reduced if the approach speed was low (on the grounds that the driver had more time to react if they misjudged, etc. plus the consequence was probably lower) or if we applied aspect controls on the previous signal to "warn the driver" and also make sure the train was going slower.
When one introduces a system such as AWS then there is an extra prompt to the driver (but we now know they can acknowledge it subconsciously).
Adding a train protection system such as TPWS changes the picture again - and we (NR) have for some time been requiring that the TPWS is effective to stop most trains using the route within the SOD (which may be longer than the overlap). Effectiveness is tested at the worst possible approach speeds so for most train approaches the speed will be lower and the train, if tripped, will stop earlier (and indeed most stop before the signal).
What that allows us to think about is removing the concept of a full overlap, a reduced overlap and a restricted overlap and designing a combined TPWS and overlap where the two combined provide the required level of protection.
Looking afresh at safety challenges and all the measures we have put in place over the years can lead to some surprising conclusions.
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