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1) the distance the front axle can get infront of the signal will depend on the location of the IBJ or the axle counter. Once the front axle passes these they will revert the previous signal to danger. For some signals these will be adjacent to the signal, for others they could only be at the end of the overlap.
2) beyond the signal there will be a distance known as an overlap which is to help reduce the effects of the a SPAD by providing a buffer for a train to stop. While a train is within this section the previous signal will not be able to be cleared.
Depends if a signal is first or last wheel replacement. You are not clearing a signal to green if a train is in the next section, that doesn't end well and the interlocking isn't going to let you do that. You won't find a map with historic journeys on them that I know of.
1) the distance the front axle can get infront of the signal will depend on the location of the IBJ or the axle counter. Once the front axle passes these they will revert the previous signal to danger. For some signals these will be adjacent to the signal, for others they could only be at the end of the overlap.
2) beyond the signal there will be a distance known as an overlap which is to help reduce the effects of the a SPAD by providing a buffer for a train to stop. While a train is within this section the previous signal will not be able to be
1) the distance the front axle can get infront of the signal will depend on the location of the IBJ or the axle counter. Once the front axle passes these they will revert the previous signal to danger. For some signals these will be adjacent to the signal, for others they could only be at the end of the overlap.
2) beyond the signal there will be a distance known as an overlap which is to help reduce the effects of the a SPAD by providing a buffer for a train to stop. While a train is within this section the previous signal will not be able to be cleared.
2) beyond the signal there will be a distance known as an overlap which is to help reduce the effects of the a SPAD by providing a buffer for a train to stop.
Most automatic signals don't have separate detection of the overlap. (Automatic signals just work themselves according to whether the line is clear, as opposed to controlled signals which can be set individually for each train).
Do you have source for this? As far as I'm aware it doesn't depend on the number of aspects the signal has, unless the signal has a semaphore distant in which case its overlap must normally be 440 yards.
Most automatic signals don't have separate detection of the overlap. (Automatic signals just work themselves according to whether the line is clear, as opposed to controlled signals which can be set individually for each train).
Do you have source for this? As far as I'm aware it doesn't depend on the number of aspects the signal has, unless the signal has a semaphore distant in which case its overlap must normally be 440 yards.
Usually 180m for automatic signals (defined in #12). For controlled signals, usually when the front of the train is a few metres beyond the signal, although it may take a second or so for the system to respond. For controlled signals with last wheel replacement, when the rear of the train is a few metres beyond the signal. But there are quite a few exceptions to this.
If first wheel replacement: when the next track circuit/axle counter section beyond the signal becomes occupied.
If last wheel replacement: when the track circuit/axle counter section before the signal becomes unoccupied.
Do you have source for this? As far as I'm aware it doesn't depend on the number of aspects the signal has, unless the signal has a semaphore distant in which case its overlap must normally be 440 yards.
Many signals in complex areas have a half-way-house between first- and last-wheel replacement: that is “delayed” replacement. The signal returns to red when the second or third track circuit becomes occupied.
This caters for locos where the cab is not at the extreme front. Steam, class 20s running bonnet first, etc, in conjunction with the first track being at the signal itself.
There is no overlap in absolute block - there is a clearing point instead, which whilst similar, has quite a few differences. It is 200 yards with a colour light distant signal and 440 with a semaphore distant signal.
Interesting that - I think - the overlap being 440 yards dates back a long time, back to when Inspecting Officers used to talk of 'protection the thickness of a signal post' and deciding to do something about it.
I wonder whether it was originally based on any scientific analysis, or just 'a quarter of a mile will be enough'.
Interesting that - I think - the overlap being 440 yards dates back a long time, back to when Inspecting Officers used to talk of 'protection the thickness of a signal post' and deciding to do something about it.
I wonder whether it was originally based on any scientific analysis, or just 'a quarter of a mile will be enough'.
A quarter of a mile in addition to the braking distance provided by the Distant signal of course, let alone any additional Stop signals provided beyond the Outer Home, which would only be cleared by the signaller when he was satisfied that the train was fully ‘under control’.
A quarter of a mile in addition to the braking distance provided by the Distant signal of course, let alone any additional Stop signals provided beyond the Outer Home, which would only be cleared by the signaller when he was satisfied that the train was fully ‘under control’.
Yes, I realise that, all those other signals had to be passed at caution or danger.
But in the days of its introduction, engines - at least on the LNWR - only had brakes (wooden blocks) on the tender and on a 10 ton brakevan (similar wood): there seemed to be only vague train load limits; signals were dimly illuminated at night (if the lamp hadn't gone out); fog was seemingly much more common than today; greasy rails from pollution; drivers were distracted by problems on the footplate and often endured very long turns of duty. (One Midland Railway loco crew were dismissed for passing a signal at danger, they had been on duty 24 hours continuous - although admittedly a lot of it may have been spent on the block - and had fallen asleep).
That's why I thought 440 yards was a bit arbitrary - if the driver hadn't managed to stop after passing the succession of signals you describe - possibly a mile or more after first sighting the distant - who knows when he would be able to stop? I guess he was trying his best! Sometimes coming down the Valleys or on the Brecon & Merthyr for instance they only stopped when they hit another train.
But, despite the improvements from those days, it's still 440 yards.
As a fun aside, in Melbourne, Australia, the overlaps are full braking distance. Combined with trainstops, it really is belt, braces and a piece of string! It does make the interlocking 'interesting'.
Yes, I realise that, all those other signals had to be passed at caution or danger.
But in the days of its introduction, engines - at least on the LNWR - only had brakes (wooden blocks) on the tender and on a 10 ton brakevan (similar wood): there seemed to be only vague train load limits; signals were dimly illuminated at night (if the lamp hadn't gone out); fog was seemingly much more common than today; greasy rails from pollution; drivers were distracted by problems on the footplate and often endured very long turns of duty. (One Midland Railway loco crew were dismissed for passing a signal at danger, they had been on duty 24 hours continuous - although admittedly a lot of it may have been spent on the block - and had fallen asleep).
That's why I thought 440 yards was a bit arbitrary - if the driver hadn't managed to stop after passing the succession of signals you describe - possibly a mile or more after first sighting the distant - who knows when he would be able to stop? I guess he was trying his best! Sometimes coming down the Valleys or on the Brecon & Merthyr for instance they only stopped when they hit another train.
But, despite the improvements from those days, it's still 440 yards.
Probably a case of inertia, plus the fact that as many of these causes of overruns became less likely, society's expectations of safety increased.
Also, as well as the train passing the clearing point the signaller had to see the tail lamp before giving train out of section, so it had to have passed the box as well*. So in many cases moving either the signal or the clearing point would have no effect on when the train could be belled out and another one offered.
*Exceptions where the train went into a loop before the box and the guard would wave the tail lamp in the direction of the box, or more recently for tail lamp cameras, but neither particularly relevant to this discussion.
As a fun aside, in Melbourne, Australia, the overlaps are full braking distance. Combined with trainstops, it really is belt, braces and a piece of string! It does make the interlocking 'interesting'.
This is often the case with Metro-type routes such as London Underground, often in conjunction with equipment that applies emergency brakes if the signal is passed at danger. Compared with the main line, speeds are lower and brakes are more effective and consistent for all trains, so the stopping distances are that much shorter.
Many signals in complex areas have a half-way-house between first- and last-wheel replacement: that is “delayed” replacement. The signal returns to red when the second or third track circuit becomes occupied.
This caters for locos where the cab is not at the extreme front. Steam, class 20s running bonnet first, etc, in conjunction with the first track being at the signal itself.
My recollection was that the replacement joint was usually at least 5m beyond the signal. 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.
Last wheel replacement was to cater for trains being propelled, where the driver would be at the back of the train. So the signal would only be replaced when the rear of the train passed the replacement joint. Last wheel replacement is rarely provided on modern schemes, as propelling is rarely permitted these days, but there are plenty of places where it still exists.
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.
== Doublepost prevention - post automatically merged: ==
Yes, I realise that, all those other signals had to be passed at caution or danger.
But in the days of its introduction, engines - at least on the LNWR - only had brakes (wooden blocks) on the tender and on a 10 ton brakevan (similar wood): there seemed to be only vague train load limits; signals were dimly illuminated at night (if the lamp hadn't gone out); fog was seemingly much more common than today; greasy rails from pollution; drivers were distracted by problems on the footplate and often endured very long turns of duty. (One Midland Railway loco crew were dismissed for passing a signal at danger, they had been on duty 24 hours continuous - although admittedly a lot of it may have been spent on the block - and had fallen asleep).
That's why I thought 440 yards was a bit arbitrary - if the driver hadn't managed to stop after passing the succession of signals you describe - possibly a mile or more after first sighting the distant - who knows when he would be able to stop? I guess he was trying his best! Sometimes coming down the Valleys or on the Brecon & Merthyr for instance they only stopped when they hit another train.
But, despite the improvements from those days, it's still 440 yards.
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.
== Doublepost prevention - post automatically merged: ==
Most automatic signals don't have separate detection of the overlap. (Automatic signals just work themselves according to whether the line is clear, as opposed to controlled signals which can be set individually for each train)
Standard Signalling Principle 20 mandated that automatic signals should normally have combined berth and overlap track-circuits, i.e. where the berth track-circuit extends beyond the signal to the end of the overlap, with no separate overlap track-circuit. Exceptions where an auto required a separate overlap track-circuit were where protecting level crossings and where permissive moves read up to the auto signal.
Although it was standard practice to provide a separate overlap track-circuit for controlled signals, this wasn't actually a requirement. SSP20 only mandated certain (most, but not all) situations. The Southern in particular was at one time quite keen on controlled signals with combined berth and overlap. As I recall, the rule of thumb we used in the 1970s was £10k per track-circuit in an electrified area, so you can understand why they would want to keep the numbers of track-circuits to a minimum. The problem, of course, is that with combined berth and overlap the signaller can't tell if a train has passed the signal or not.
SSP20 also permitted combined berth and overlap for restricted overlaps (where the overlap is only 50 yards long).
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.
The Southern used this quite extensively on some of the early stages of the Victoria resignalling, using reed track-circuits. Unfortunately the reed jointless track-circuits proved problematic with DC electrification, and so some time after commissioning they rapidly had to be converted to jointed. It wasn't practicable at that stage to provide additional track-circuits for the overlaps, so many of the controlled signals ended up with combined berth and overlap.
As many have said about the many differences in overlaps and clearance points over the years and different networks for your second question, I'll add to this one. If anything is in the overlap of the route from signal, for example 1 to 3* (so the overlap of this route is after signal 3) the route cannot be set from signal 1 until the overlap is clear. This means even when a combined berth and overlap a train will be held at the pervious signal. As others have said overalp is a safety margin. The very rare exception for an overlap to not be clear, and it is really not the overlap but the first track section** is called something like standing ahead controls where at a terminus, you occasionally have a long train that for whatever reason (such as when the loco is added) doesn't fit inside the starting signal. There will be added controls in the interlocking for this and indictors so the train crew can see. I believe Inverness has on for the Sleeper and Euston used to have them.
*signals are generally numbered in the GB network to be odd in one direction and even in the other, so, if a two track railway Signal 1 could be followed by 3. A major exception is in an area operated by mechanical levers or Individual Function Switch Panels, where they are numbered to match the lever or switch
** Getting into signalling design weeds here, but one track section doing two things: take this as a track layout being one direction of of a double track with no bi directional working. Using text characters (sorry if it dispalys oddly) with = being track O being the signal and ] being the overlap:
=O=]===O=]===O=]
Sig.1 Sig.3 Sig.5
The route from signal 1 to 3 has an overlap beyond signal 3, but this is also the first track section of the route from signal 3 to 5. So a train standing on that track will prevent both signal 1 and 3 being cleared.
I believe that some of the sites like Realtimetrains allow looking into the past (some with subscription), but I don't know of any with map view. The industry, on newer installations have an equivalent OTMR/Airline black box for the signalling that can be replayed but obviously not for external users
The route from signal 1 to 3 has an overlap beyond signal 3, but this is also the first track section of the route from signal 3 to 5. So a train standing on that track will prevent both signal 1 and 3 being cleared.
You know, it is just a word/term I use, I have not really though about its origins. You could well be right and why the name change from absolute block. I wonder if anyone knows!
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