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ideally, but there are some places where the joint is much closer. Sometimes necessary in tight throats where the joint also forms a clearance joint for point deadlocking.
Where the joint was closer, so there was a chance of the driver seeing the signal go back, then delayed replacement would be considered. So the signal would be replaced when the front of the train occupied the second track section.
The need for these controls would be specified at the scheme design stage, along with things like the maximum train length that should be allowed for. It was usual to be specified on the signalling scheme plan, either as a generic note, and/or notes against individual signals or routes. As others have said, these plans aren't in the public domain.
Sometimes old scheme plans, locking sketches, etc turn up for sale, but not for live installations, clearly. I have a stack of South Wales plans in my garage, but it's a snapshot from 40 years ago, none of it current!.
Yes, the 440 yards is/was an entirely arbitrary distance. It was intended to cover a driver misjudging his braking, and slightly over running the stop signal. It was never intended to cover a driver completely ignoring or failing to see the distant signal, for example.
Early colour light signalling schemes used 440 yard overlaps, but with regular, relatively closely-spaced signals, it was soon found that this length of overlap had a significant impact on headways. An analysis of overruns in colour light areas showed that those due to misjudgement were well within 200 yards, so the HMRI were persuaded to accept 200 yard overlaps for colour-light signalling. But the clearance point for semaphore signalling remained at 440 yards.
BR did further analysis, and SSP 20 permitted shorter overlaps for colour-light signalling depending on attainable train speed, down to a minimum of 50 yards at 15mph, where necessary. But 200 yards was still the norm. They also regularised the situation for block signalling using colour-lights. Interestingly, this attainable speed was initially based on a seemingly arbitrary distance of 440 yards on the approach to the signal, although this was later changed to the speed at the first caution.
A common Southern trick in tight throats where the restricted overlap joint was also the replacement joint, with an alternative full overlap extending further through junction pointwork.
Some types of jointless track-circuits allowed track-circuits to be overlapped. Normally one track-circuit ends at a set of insulated joints, and the next begins immediately after. However, because jointless track-circuits don't have insulated joints, the start of one track-circuit doesn't necessarily have to align with the end of the previous - you can overlap them. So you can make the berth track-circuit extend overlap distance beyond the signal, while the next section track-circuit commences immediately beyond the signal. So any vehicle standing in the overlap will be detected by both track-circuits. This again saves having to provide a separate overlap track-circuit.
Yes, the 440 yards is/was an entirely arbitrary distance. It was intended to cover a driver misjudging his braking, and slightly over running the stop signal. It was never intended to cover a driver completely ignoring or failing to see the distant signal, for example.
/snip
I have an interest in the causes of accidents and spend a lot of time looking through old accident reports on Railways Archive.
In the days before 'Overlap' was adopted there were a lot of collisions the result of a signal being passed at danger, for a while variety of reason.
The Inspecting Officer often referred to the distance to the fouling point from the stop signal in these accidents. At least once, the point was made that, logically, because there was no specification of the minimum distance to the fouling point the safety margin could be the thickness of the signal post,.
So there was strong pressure from the Inspecting Officer to introduce a standard longer distance to the fouling point, but, like other safety changes such as Absolute Block or automatic brakes, he relied on the good will of the companies to adopt these ideas.
But 'Overlap' was adopted.
Although Railways Archive has a good selection, under 'Documents' of railway acts and regulation, I haven't been able to find any reference to the requirement for what is now known as overlap, which is strange. If any document could be traced it might give the reasons for the choice of a quarter mile.
One of the reasons for my suggestion that 440 yards seems to be entirely arbitrary is that, on a steeply graded line (eg ex LNWR to Buxton) the overlap is the same on a steep climb as on a similarly steep descent. It would be interesting to track downing document that gave the origin of the choice of a quarter mile.
Although both freight and passenger trains took a much greater distance to stop then than now the speeds were, generally, slower so perhaps 440 yards is still appropriate.
I am using 'fouling point' generally to mean obstruction - not just where another line converges trailing with the running line in question.
I seem to recall that initially distant signals were required to be a nominal one mile from the first stop signal. Nowadays the distance between the first caution signal is based on braking distance, which is calculated based on speed and gradient. But the overlap length is still an arbitrary distance regardless of gradient. The braking distance will increase on a falling gradient, but the length of the overlap won't.
The HMRI requirements were initially an informal set of guidelines that the BoT inspecting officers would use when approving a new railway, that were subsequently formalised and published. These requirements were very detailed, and I would have thought some mention would be made in them about the clearing point. The National Archives in Kew might have correspondence about how they developed.
But the overlap length is still an arbitrary distance regardless of gradient. The braking distance will increase on a falling gradient, but the length of the overlap won't.
While there is no specific consideration of overlaps (although gradients on approach would affect the attainable speed, which definitely does adjust the overlap length), doesn't this section of the standard require it to be considered?
The design of each overlap shall be optimised to reflect the following constraining influences:
...
f) the influences on driving while making the journey towards the exit signal:
...
• the effect of significant gradients on the immediate approach (within braking distance) to the exit signal;
I seem to recall that initially distant signals were required to be a nominal one mile from the first stop signal. Nowadays the distance between the first caution signal is based on braking distance, which is calculated based on speed and gradient. But the overlap length is still an arbitrary distance regardless of gradient. The braking distance will increase on a falling gradient, but the length of the overlap won't.
The HMRI requirements were initially an informal set of guidelines that the BoT inspecting officers would use when approving a new railway, that were subsequently formalised and published. These requirements were very detailed, and I would have thought some mention would be made in them about the clearing point. The National Archives in Kew might have correspondence about how they developed.
Looking at very old accident reports it seems that the term 'distant signal' had a different meaning, and drivers were expected to stop at them - GW and South Wales railways I remember.
Possibly the original meaning of 'distant signal' was 'the one further away' and 'home' was the one nearby and both similar in terms of indication meaning - but that's just a guess. They were both painted red for a long time.
In older accident reports there's always a vey detailed examination of distances between signals & to 'box, all to the nearest yard; although the relevance is not always clear to me.
Referring back to my previous post about the origin of the term, have you seen "overlap" used in the context of Absolute Block signalling? I've also read a lot of accident reports and don't recall it anywhere - the common term being "clearing point" for the end of the 440 yards.
My suggestion of it coming from "overlapping" routes wouldn't work with AB, because "routes" weren't a thing and the line was split up into "station limits" between the first and last stop signal controlled by a box, and "block section" from the last stop signal to the first stop signal of the next box. At least I think that's the definition - maybe the block section ran up to the clearing point - but the current signalling glossary doesn't include any of these terms and I don't have anything else to hand to check with.
There was also the warning arrangement, used in AB where the line was clear only up to the first stop signal with no margin for error. Trains were slowed nearly to a stop at the previous box and sent forward with a handsignal or occasionally a special signal arm. The modern equivalent is a warner route, approach controlled from yellow for a short overlap.
Referring back to my previous post about the origin of the term, have you seen "overlap" used in the context of Absolute Block signalling? I've also read a lot of accident reports and don't recall it anywhere - the common term being "clearing point" for the end of the 440 yards.
My suggestion of it coming from "overlapping" routes wouldn't work with AB, because "routes" weren't a thing and the line was split up into "station limits" between the first and last stop signal controlled by a box, and "block section" from the last stop signal to the first stop signal of the next box. At least I think that's the definition - maybe the block section ran up to the clearing point - but the current signalling glossary doesn't include any of these terms and I don't have anything else to hand to check with.
There was also the warning arrangement, used in AB where the line was clear only up to the first stop signal with no margin for error. Trains were slowed nearly to a stop at the previous box and sent forward with a handsignal or occasionally a special signal arm. The modern equivalent is a warner route, approach controlled from yellow for a short overlap.
The Railways Archive has a word search facility and I've tried using it to search for 'Overlap'. That produces many hundreds of matching documents but unfortunately does not highlight where the word occurs in a document.
It does return the accident at Stretton (Derbyshire) October 1875 as containing the word "overlap' but I can't find that word in it. However, it does include the following, in the report by the Investigating Officer :-
The interval of space, the preservation of which is so essential to safety, and is, indeed, the aim and object of block-working, becomes thus reduced, so to speak, to the thickness of the signal-post.
I need to look further to see whether 'interval of space' is used in other reports - suggesting the official term for what's now called overlap - or whether it was just a turn of phrase used once.
I don't know whether this is the only report to contain 'thickness of the signal post', a remark that's often quoted in (non-official) books about signalling.
The Railways Archive has a word search facility and I've tried using it to search for 'Overlap'. That produces many hundreds of matching documents but unfortunately does not highlight where the word occurs in a document.
It does return the accident at Stretton (Derbyshire) October 1875 as containing the word "overlap' but I can't find that word in it. However, it does include the following, in the report by the Investigating Officer :-
The interval of space, the preservation of which is so essential to safety, and is, indeed, the aim and object of block-working, becomes thus reduced, so to speak, to the thickness of the signal-post.
I need to look further to see whether 'interval of space' is used in other reports - suggesting the official term for what's now called overlap - or whether it was just a turn of phrase used once.
I don't know whether this is the only report to contain 'thickness of the signal post', a remark that's often quoted in (non-official) books about signalling.
Could be a reference to "space interval working", the block system as we now understand it, in contrast to "time interval working" which the railways originally used.
Could be a reference to "space interval working", the block system as we now understand it, in contrast to "time interval working" which the railways originally used.
Thanks, that would explain it - there were extracts from the rules under the 'block-telegraph-system' and reference to when 'line clear' could be given so clearly the block system much as we know it now was in operation.
Looking at very old accident reports it seems that the term 'distant signal' had a different meaning, and drivers were expected to stop at them - GW and South Wales railways I remember.
You need to remember that semaphore signalling and Absolute Block Working as we now know them weren't suddenly invented, they came about through a long process of evolution. Also that although the BoT had the power to refuse approval for a new railway to be opened (to passengers), they had no authority over existing railways. Once they had given approval for a railway to open, that was it. So many railways carried on using outdated methods long after new lines proposing the same would be refused permission (for example, the Ffestiniog Railway continued using its antiquated rotating disc signals and slotted post signals right up until closure in the 1940s). If there were an accident, the inspectors would rail against companies continuing to use these outdated systems, but they had no power to mandate changes.
In my previous post, I mentioned the "HMRI Requirements". Some of this was incredibly specific (e.g. the block bells must be audible from the toilet). But some of it was very generic, allowing the inspectors a lot of flexibility in interpretation. In particular, the requirement for block working:
"Apparatus to be installed for ensuring, by means of the block telegraph system or by other approved method,..., an adequate interval of space between following trains.....".
Note that there is no mention of clearing points or overlaps. But if not provided, the inspectors would rule that the requirement had not been met. And just a requirement for "an adequate interval of space" - no mention of 440 yards.
So, where did the 440 yards come from? I suspect that it was a figure that the BoT inspectors used, 440 yards being their interpretation of "adequate". The inspectors could and did interprete words like "adequate" and "suitable" to set de-facto standards. For example, different companies initially used very different systems of block and telegraph working. However, the inspectors were able to standardise this to a great extent, not by explicit printed requirements in the guidance, but purely by what they deemed adequate. Rather than looking at accident reports, it might be more instructive to look at BoT/HMRI reports of new lines being inspected at about the time block working became compulsory, to see what the inspectors were saying about what they considered acceptable and what not.
The RCH later standardised absolute block working even further. So the RCH may have standardised on 440 yards. Although I suspect that for them to do so, it must already have been commonly accepted practice.
== Doublepost prevention - post automatically merged: ==
Referring back to my previous post about the origin of the term, have you seen "overlap" used in the context of Absolute Block signalling? I've also read a lot of accident reports and don't recall it anywhere - the common term being "clearing point" for the end of the 440 yards.
I have a 1940s text book ("Railway Signalling and Communications") that refers to the "block overlap" when discussing traditional semaphore signalling, so it is clear that such confusion is not new!
== Doublepost prevention - post automatically merged: ==
It does return the accident at Stretton (Derbyshire) October 1875 as containing the word "overlap' but I can't find that word in it. However, it does include the following, in the report by the Investigating Officer :-
The interval of space, the preservation of which is so essential to safety, and is, indeed, the aim and object of block-working, becomes thus reduced, so to speak, to the thickness of the signal-post.
I need to look further to see whether 'interval of space' is used in other reports - suggesting the official term for what's now called overlap - or whether it was just a turn of phrase used once.
Thanks, that would explain it - there were extracts from the rules under the 'block-telegraph-system' and reference to when 'line clear' could be given so clearly the block system much as we know it now was in operation.
That is a very big assumption. There were some VERY different systems of block working. Most early block instruments only had two indications, not the three that are usual today. There were single-wire instruments, where the indication was operated by the bell plunger. And some early instruments even only had one single needle for both up and down lines!
The compulsory provision of quarter-mile posts required by the Railway Clauses Act of 1845 gave a regular measure of 440yards. Could this have resulted in this distance being used for the clearing point distance?
While there is no specific consideration of overlaps (although gradients on approach would affect the attainable speed, which definitely does adjust the overlap length), doesn't this section of the standard require it to be considered?
Ah, one of the problems with getting out of date with current practice. The overlap lengths specified in SSP20 and its successor standards didn't include gradient in any consideration of overlap length. It looks like the standard does now include it as "an influence". But quite how do you decide that something that is purely arbitary in the first place is or isn't good enough?!
Unfortunately this is symptomatic of what has been happening to the standards since privatisation, if not before. Rather than providing genuine guidance on what to do, you are now required to risk assess each situation, consider potential influences, etc etc. It is pure backside covering - rather than take the responsibility of providing genuine helpful guidance, the responsibility is all piled onto the person trying to follow the guidance.
The compulsory provision of quarter-mile posts required by the Railway Clauses Act of 1845 gave a regular measure of 440yards. Could this have resulted in this distance being used for the clearing point distance?
I somewhat doubt it, probably just choosing a usefully large but still memorable number. If your home signal was nowhere near a quarter-mile post it wouldn't be easy to work out where the quarter mile was. I believe, often there was just an object used that was used by (told to?) signalmen to judge the 440 distance.
I suspect they often put the first stop signal 440 yards on the approach to the signal box, so the signaller could give train out of section as soon as they saw the tail lamp.
I somewhat doubt it, probably just choosing a usefully large but still memorable number. If your home signal was nowhere near a quarter-mile post it wouldn't be easy to work out where the quarter mile was. I believe, often there was just an object used that was used by (told to?) signalmen to judge the 440 distance.
The signalbox instructions should state where the clearing points are - the signalman isn't expected to work it out for themselves. Likewise it should state the fogging point, something that if it isn't visible from the signalbox due to fog then the fogging regulations come into force.
The signalbox instructions should state where the clearing points are - the signalman isn't expected to work it out for themselves. Likewise it should state the fogging point, something that if it isn't visible from the signalbox due to fog then the fogging regulations come into force.
Yes I did just write originally 'told' but was trying to be vague to avoid the "well actually on the No Where Railway in 1896 the Signalmen did it this way..." type comment and made the statement too loose
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