Definition of absolute block signalling

[eldomtom2]

Is there an official definition of absolute block signalling? Wikipedia, citing Ellis' British Railway Engineering Encyclopaedia, defines it as "a British signalling block system designed to ensure the safe operation of a railway by allowing only one train to occupy a defined section of track (block) at a time", but that would seem to include other forms of signalling as well.

 

[Gaelan]

Others will have a better sense than me, but I think the defining characteristic is that a staffed signal box sits at each block boundary, with the signaler clearing the signal when they personally observe the train leave the block, as opposed to track circuits or similar.

 

[Gloster]

I leave you to develop a detailed definition, but it is the physical observation of the train by the signalman and his or her observation of the rear of train/train complete marker (a.k.a. the tail-light) as it passes him or her that the train is complete and the section is clear and a subsequent train may be allowed by the signalman, in cooperation with the signalman at the previous box, to enter the section. All other gubbins add to the safety, but that is the fundamental point.

 

[ComUtoR]

One train, in one block section, at any one time.

That's the definition I learned.

 

[12LDA28C]

One train, in one block section, at any one time.

That's the definition I learned.

Indeed, however I would like to think that also applies to Track Circuit Block signalling. I believe that's the thrust of the OP's question.

 

[Railsigns]

One train, in one block section, at any one time.

That's the definition I learned.

That's the definition of space interval working, as distinct from time interval working. Absolute block is one method of working based on intervals of space, but there are others.

 

[ComUtoR]

Indeed, however I would like to think that also applies to Track Circuit Block signalling. I believe that's the thrust of the OP's question.

Definitions taken from the RSSB Rule Book Glossary

Train Signalling Regulations
Absolute block: A signalling system that allows only one train to be in a block section at the same time. The block indicator is used to indicate whether the line between adjacent signal boxes is clear or occupied.

Track Circuit block: A method of signalling trains in a section of line using track circuits or other means of automatic train absence detection and without using block instruments.

 

[etr221]

My understanding is that 'The Absolute Block System' (in British practice) is that it provides for the passage of trains through block sections between signal boxes, which communicate with the use of block instruments (a specialised form of electric telegraphy) to ensure only one train is in the section at a time. And it is the use of block instruments which is the defining characteristic: other systems enhance them, or replace them by continuous track circuiting, or other form of train-in-section sensing, that obtain the same 'one train in block section at a time' end, which is a more general definition of absolute block working, but have different names.

 

[Gloster]

But always remember that without the human beings observing the trains and operating the instruments the system does not function.

 

[Railsigns]

But always remember that without the human beings observing the trains and operating the instruments the system does not function.

Observing the trains may not be necessary in cases where the block section has continuous train detection.

 

[Ken H]

To me it means 'not permissive block or time interval working'
Not sure if moving block qualifies....

 

[John Webb]

Looking at the history of signalling to explain things to visitors at the preserved St Albans South Signal Box, it seems that the Board of Trade brought in the Term "Absolute Block" when they insisted on everyone moving away from the 'Time Interval' or 'Open Block' system. In the latter, signals were cleared behind trains after so many minutes had passed. This led notably to the Abbot's Ripton crash where signals in a winter storm froze in the 'clear' position and did not go back to caution or danger when the signalman thought he'd operated them.
Basically 'Absolute Block' therefore refers to:
1. Signals being left at caution or danger until a signalman and his colleague in an adjacent box agree (by use of the block instruments) a train shall pass from one to the other.
2. Signals are replaced to caution or danger as soon as a train has passed them.
3. The observation of a tail light to confirm the whole train has safely passed from one box to the next.

Track circuits assist the operation of the Absolute Block system by helping the signaller to understand where the trains are and therefore when signals can be replaced to danger or caution. They also serve to remind the signalman of trains standing on the line (eg the Hawes Junction/Aisgill incident where two light locos were overlooked by the signalman.)

 

[Ken H]

Looking at the history of signalling to explain things to visitors at the preserved St Albans South Signal Box, it seems that the Board of Trade brought in the Term "Absolute Block" when they insisted on everyone moving away from the 'Time Interval' or 'Open Block' system. In the latter, signals were cleared behind trains after so many minutes had passed. This led notably to the Abbot's Ripton crash where signals in a winter storm froze in the 'clear' position and did not go back to caution or danger when the signalman thought he'd operated them.
Basically 'Absolute Block' therefore refers to:
1. Signals being left at caution or danger until a signalman and his colleague in an adjacent box agree (by use of the block instruments) a train shall pass from one to the other.
2. Signals are replaced to caution or danger as soon as a train has passed them.
3. The observation of a tail light to confirm the whole train has safely passed from one box to the next.

Track circuits assist the operation of the Absolute Block system by helping the signaller to understand where the trains are and therefore when signals can be replaced to danger or caution. They also serve to remind the signalman of trains standing on the line (eg the Hawes Junction/Aisgill incident where two light locos were overlooked by the signalman.)

Of course the track circuits can lock levers to stop conflicting routes being set up..
Part of absolute block is the rule that a train stopped at a stop signal must go to the signal box, remind the signalman about his train, check lever collars are in place and sign the train register. If there is a track circuit then there will be a lozenge white plate on the signalpost yo exempt that signal from the need for the driver to visit the box. I think its OK to use a signal post telephone. I am not sure how this has changed for todays railway.

 

[edwin_m]

Of course the track circuits can lock levers to stop conflicting routes being set up..
Part of absolute block is the rule that a train stopped at a stop signal must go to the signal box, remind the signalman about his train, check lever collars are in place and sign the train register. If there is a track circuit then there will be a lozenge white plate on the signalpost yo exempt that signal from the need for the driver to visit the box. I think its OK to use a signal post telephone. I am not sure how this has changed for todays railway.

That isn't unique to absolute block. The rule to contact the signaller applies under all forms of working, although with track circuit block it will be via the signal post telephone or by radio.

 

[Annetts key]

It's important to understand that with absolute block signalling, the signal box is the "block post". With track circuit block or other types that use technology, one signal box (block post) can control or supervise many track circuit block sections.

BTW, a crossing keepers box is not a "block post" hence why they are not normally officially called signal boxes, even though they may control signals.

Pinning down an exact definition is tricky, because the various different railway companies in Britain all had their own variations on absolute block signalling.

Just look at the pictures of the various different block instruments. Although most use "Train On Line / Line Blocked / Line Clear", the actual operation is not exactly the same for each block manufacturer/railway company/region.

The principle is however the same. To prevent more than one train being in the 'block' section between 'block posts'.

 

[Elecman]

Google and watch the LMS Film Sentinels of Safety

 

[Greybeard33]

Is not the concept of station limits an essential part of the Absolute Block system? Along with the distinction between a home signal and a starter signal?

 

[Railsigns]

Is not the concept of station limits an essential part of the Absolute Block system? Along with the distinction between a home signal and a starter signal?

It's possible for a block post to have no station limits, in which case the home signal is also the starting (section) signal.

 

[edwin_m]

Another confusion are intermediate block signals, which may be added to increase capacity by splitting a long block section or to abolish a signal box that doesn't need to control points or a level crossing. Because there's nobody at the IB signal to observe the tail lamp, the section leading up to it must be fully track circuited. So it becomes effectively a small piece of track circuit block, though it's not described as such.

An Absolute Block box with multiple stop signals is the only case where a driver can see a green aspect followed by a red with no intervening yellow aspect or distant board. This is only allowed with semaphores (if the signal in question is a colour light it will have a yellow) and the train should have been brought nearly to a stand before the first signal is cleared.

 

[Railsigns]

An Absolute Block box with multiple stop signals is the only case where a driver can see a green aspect followed by a red with no intervening yellow aspect or distant board. This is only allowed with semaphores (if the signal in question is a colour light it will have a yellow) and the train should have been brought nearly to a stand before the first signal is cleared.

That situation isn't restricted to Absolute Block. You can find it on lines worked by Electric Token Block or Tokenless Block, and even Track Circuit Block.

 

[Rescars]

Should we also mention the enhancements to Absolute Block created by Sykes Lock and Block which, IIRC, prevented a starter signal being pulled off until the previous train had cleared the section ahead and released the signal by means of a treadle.

Just look at the pictures of the various different block instruments. Although most use "Train On Line / Line Blocked / Line Clear", the actual operation is not exactly the same for each block manufacturer/railway company/region.

..... and just to add to the complexity, AIUI the GWR preferred the term "Normal" instead of "Line Blocked".

 

[Islineclear3_1]

Is not the concept of station limits an essential part of the Absolute Block system? Along with the distinction between a home signal and a starter signal?

And the concept of "clearing point?"

 

[norbitonflyer]

My understanding was that Absolute Block meant any system which requires that only one train can be in a section at any one time - however a section is defined. Track Circuit Block (whether continuous or otherwise) is one system of ensuring that happens. The Sykes system was another.

If it is not Absolute, it is Permissive Block. The line is still divided into sections, but under certain circumstances a second train may enter a section that is occupied, with the driver cautioned accordingly. As I understand it, it is normally only allowed for goods lines, or when necessary within station limits to allow attachment of locomotives or carriages to trains.

 

[Gloster]

I think that the vital point is that Absolute Block mandates only one train at a time in one section, which is usually between different boxes and their most advanced Starting and outermost Home signals. The proof that a train has passed through the section (and that another can be allowed to enter it) is by the physical observation of the taillight and not by track-circuits, axle counters or any other technical system. Track-circuit block differs because it is track-circuits, etc., that indicates whether a train has passed clear of a section of track, which is between two signals. It is a development on the same lines (pun intended) of AB, but is not the same thing.

 

[Railsigns]

I think that the vital point is that Absolute Block mandates only one train at a time in one section, which is usually between different boxes and their most advanced Starting and outermost Home signals. The proof that a train has passed through the section (and that another can be allowed to enter it) is by the physical observation of the taillight and not by track-circuits, axle counters or any other technical system.

Then how do you categorise those sections that are defined as Absolute Block in the Sectional Appendix and are provided with ordinary block instruments, but don't require the physical observation of tail lamps because there's continuous track circuiting through the block section?

 

[Gloster]

Then how do you categorise those sections that are defined as Absolute Block in the Sectional Appendix and are provided with ordinary block instruments, but don't require the physical observation of tail lamps because there's continuous track circuiting through the block section?

Because there are all sorts of oddities on the railway and a half-and-half system like this has to be classified as something. Do you create a special new clsassification for something like this or do you play safe and call it the most basic system? Do the box instructions say that the signalman doesn’t have to check the taillight?

 

[Railsigns]

Because there are all sorts of oddities on the railway and a half-and-half system like this has to be classified as something. Do you create a special new clsassification for something like this or do you play safe and call it the most basic system? Do the box instructions say that the signalman doesn’t have to check the taillight?

In the particular case I'm thinking of, the box instructions allow the signaller to send 'train out of section' when the train has passed clear of a certain track circuit, which is well in rear of the signal box. The signaller is required to observe each train as it passes, "where possible".

The main thing that sets Absolute Block apart from all other non-permissive methods of working double/multiple track railways is the use of block instruments to record the state of the section.

 

[Ken H]

In the particular case I'm thinking of, the box instructions allow the signaller to send 'train out of section' when the train has passed clear of a certain track circuit, which is well in rear of the signal box. The signaller is required to observe each train as it passes, "where possible".

The main thing that sets Absolute Block apart from all other non-permissive methods of working double/multiple track railways is the use of block instruments to record the state of the section.

So what to block instruments show with intermediate block?

Also of imterest here is the arrangements when switching boxes out, say at night, and ensuring that only one train can be in the longer section during the switching out procedure.

Also the arrangements where absolute block controlled by a signalman with his blick instruments, bells and levers joins onto an track circuit block controlled section. Hellifield to York power box at Skipton is a good example.

 

[Tomnick]

That situation isn't restricted to Absolute Block. You can find it on lines worked by Electric Token Block or Tokenless Block, and even Track Circuit Block.

Swinderby is an excellent example – almost the textbook Absolute Block layout (until the crossover was removed at least), but actually Track Circuit Block both ways. I hear so much nonsense ("a little island of Absolute Block", "it's got semaphores so it must be AB", etc.)

Then how do you categorise those sections that are defined as Absolute Block in the Sectional Appendix and are provided with ordinary block instruments, but don't require the physical observation of tail lamps because there's continuous track circuiting through the block section?

Or the block section between Dinting and Manchester East (now a workstation in the ROC) with two "train arrived complete" plungers for the latter!

== Doublepost prevention - post automatically merged: 11 Feb 2024 ==

So what to block instruments show with intermediate block?

The same, but the block section doesn't start until the intermediate block home signal so you don't *need* a line clear from the box in advance to let a train up to that signal.

Also of imterest here is the arrangements when switching boxes out, say at night, and ensuring that only one train can be in the longer section during the switching out procedure.

You can't have a train in the section on either side, on either line, when you're switching out – presumably for that reason.

You can switch in with a train in section, but the same risk doesn't exist – you know there's one train in the longer section, and once you've switched in, there's one train in one of the two shorter sections, you just don't know which one (unless you've just seen it pass) so you don't knock out until the box in advance does or you see the train pass.

Also the arrangements where absolute block controlled by a signalman with his blick instruments, bells and levers joins onto an track circuit block controlled section. Hellifield to York power box at Skipton is a good example.

Nothing complicated. You work AB one way and TCB the other, as simple as that.

 

[Railsigns]

Swinderby is an excellent example – almost the textbook Absolute Block layout (until the crossover was removed at least), but actually Track Circuit Block both ways.

Thanks; I've just looked out the signalling plan for Swinderby. It perfectly illustrates the point.

I hear so much nonsense ("a little island of Absolute Block", "it's got semaphores so it must be AB", etc.)

Sigh...