Definition of absolute block signalling

[Gloster]

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.

Intermediate block (IB) is a bit like a very advanced Starting Signal. The line is track-circuited from the signal box to the IBS, and usually with an overlap beyond it. The first train is offered by the signal box A to signal box C, when accepted the signals at A and IBS B, which has its own (usually automatic) Distant, are cleared. When the train passes A the signalman gives Train Entering Section to C and puts the signals back to Danger: firstly those around the box and later the IBS once it has automatically gone back to Danger as the train passed. The train is in the section to C, but the signalman can now let a train up to the IBS if the overlap is clear as it is prevented from running into the back of the first train by the IBS at Danger. Once the first train passes C and the signalman there gives Out of Section, the signalman at A can ask the road for the train approaching the IBS and clear the IBS when he gets Line Clear, before giving Entering Section. When I operated an IBS we gave Entering Section as the first train passed the A, but immediately after getting Line Clear if the train was between A and the IBS when we got Line Clear.

Switching out is pretty well identical to normal switching out: only done when there are no trains either On Line or Offered. Just remember that the IBS has to be cleared as well.

At a box that is AB one side and TCB the other, the line will be entirely track-circuited on the panel side of the manual box (right up to it) and normal AB the other way. The signalman just acts as an interface by describing trains using bell codes and any method a panel may use: sometimes he or she will hear and acknowledge four beats, and then tap 1??? into a train describer.

N.b. My signalling days were thirty-five years ago.

 

[etr221]

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?

My understanding is that under 'The Absolute Block System' the line 'within station limits' (again, normally AIUI, between the home ('enter station') and starter ('leave station') signals) is not included within the block sections (which are between stations), but are under the control of (soleley) the local signal box.

So, under ABS, a train travelling A-B-C (which are stations/block posts with signal boxes):

• from A to home signal at B: block section A-B, signallers at A & B co-operate using Block Instruments to determine if line is clear and permit passage of train
• through station B, from home to starting signal: local control by signaller at B, who can check if line is clear through station
• from starting signal at B to C: block section B-C, signallers at B & C co-operate using Block Instruments to determine if line is clear and permit passage of train

Note: it is possible for the signaller at B - using his station's track layout - to receive a train from A, recess it onto an alternative track, then permit the passage of our A-B-C train and finally send the first train on to C (once the block onwards is clear)

With the passage (arrival and departure), and completeness, of trains being determined by signallers - by sight (or otherwise).

While this is an ideal model, the actual layout on the ground may require amendments, while still adhering to the 'one section, one train' principle.

The development of continuous train monitoring and remote control techniques (by track circuits or otherwise); and the decline of the traditional station - locations with side tracks, points and crossings, where the operation of trains could be rearranged - in favour of halts - just stopping points with platforms on plain line from an operational perspective, even if they have substantial facilities for passengers - have led to more modern signalling systems, which while applying the absolute block principle, are very different from 'The Absolute Block System', with different names, and matching a different model of the railway.

Edit: added reference to 'different model'

 

[Railsigns]

Regarding station limits being an essential part of Absolute Block, in the early days of Track Circuit Block it was practice to define station limits for signal boxes that controlled large areas of track under TCB principles.

 

[Annetts key]

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.

Track circuit block with route relay or computerised interlocking can be more complex than Absolute Block. A track circuit block section can both be permissive and non-permissive with the signalling system automatically switching between 'modes'. Or the signaller may have a control to select the 'mode'.

I presume the example you are thinking of is at a station where locos may couple to coaching stock or multiple unit trains may couple together. If the platform track circuit is clear, it's non-permissive and the protecting signal clears to a main aspect. However, if the track circuit for the platform is occupied, it's permissive, the protecting signal clears to a calling on/position light aspect. And yes, this is for in service passenger trains.

For non-passenger trains, signals/track circuit block sections may be permissive. Goods loops being a good example. It's still a track circuit block section, but the signalling may be designed to allow multiple goods trains into the loop (although the second and third etc. will be with a calling on/position light aspect).

Track circuit block also allows trains to run in either direction (if included as part of the design), or to reverse direction (again, if included in the design). Both of these not being possible with an Absolute Block system.

I can't say I have ever heard of anyone calling any track circuit block system by the term absolute block. Track circuit block being specificity designated as a separate and different system.

 

[Railsigns]

Track circuit block also allows trains to run in either direction (if included as part of the design), or to reverse direction (again, if included in the design). Both of these not being possible with an Absolute Block system.

It is possible to work single or bi-directional lines by interlocked AB-type block instruments, but it was usually only done over short distances or as a temporary measure.

 

[Annetts key]

It is possible to work single or bi-directional lines by interlocked AB-type block instruments, but it was usually only done over short distances or as a temporary measure.

Every day is a learning day. Did not know that.

 

[Ken H]

Re post 32 https://www.railforums.co.uk/threads/definition-of-absolute-block-signalling.262582/post-6633412
We also have to consider overlaps. The distance beyond a stop signal that must be clear before the previous signal is clear. Or the slightest SPAD could cause a major crash.

 

[matchmaker]

You can also have more than one IB section between two signal boxes - for example between Blair Atholl and Dalwhinnie there are two in each direction, replacing the signal boxes at Dalanraoch and Dalnaspidal.

 

[Rescars]

In some locations it was possible, during times of light traffic, to create long sections by merging block sections together, switching out the box between two regular sections.

 

[rower40]

When I did a 2-day signalling appreciation course in the 1990’s, I was told that the fundamental difference was that an AB signaller has a over-arching right of refusal to prevent trains being sent towards their box. Whereas a TCB signaller would have to invoke an emergency (e.g. the box to box alarm) to prevent a train entering their area. Especially if there’s a long sequence of auto signals before the first controlled signal.

 

[Gloster]

When I did a 2-day signalling appreciation course in the 1990’s, I was told that the fundamental difference was that an AB signaller has a over-arching right of refusal to prevent trains being sent towards their box. Whereas a TCB signaller would have to invoke an emergency (e.g. the box to box alarm) to prevent a train entering their area. Especially if there’s a long sequence of auto signals before the first controlled signal.

That is roughly true. The signalman at A has to get the permission of the signalman at B before s/he can clear his/her signal to allow a train to go from A to B. This is normally done by B replying to A’s bell signals and putting the Block Indictaor to Line Clear, which nowadays releases a lock on A‘s most advanced Starting Signal. In TCB trains will pass directly from A’s area to B’s automatically unless B tells A to stop them.

 

[Railsigns]

When I did a 2-day signalling appreciation course in the 1990’s, I was told that the fundamental difference was that an AB signaller has a over-arching right of refusal to prevent trains being sent towards their box. Whereas a TCB signaller would have to invoke an emergency (e.g. the box to box alarm) to prevent a train entering their area.

That distinction falls down in situations where a TCB signaller has to give a slot release to allow a train to enter their area.

 

[Annetts key]

That distinction falls down in situations where a TCB signaller has to give a slot release to allow a train to enter their area.

Such as certain single lines signalled as TCB. Certain reversible lines that are TCB, etc...

 

[LBMPSB]

It is a nineteenth century invention developed to move trains between two locations safely. It was the only way then to detect the presence of trains, by Signalmen using Rules procedures to ensure only one train was travelling towards them from the Signal Box in the rear, preventing the Signalamn in rear from sending another train towards them until the previous train had cleared the section and passed the signalbox complete, indicated by the trains red tail lamp. The telegraph system was developed to allow two Signalmen to communicate to each other by tapping out a series of bell codes as a message. Developed before the existance of the telephone. Hence why the dangerous practice of Time Interval Working existed when the telegraph/bell-code system between boxes failed.

 

[edwin_m]

It is a nineteenth century invention developed to move trains between two locations safely. It was the only way then to detect the presence of trains, by Signalmen using Rules procedures to ensure only one train was travelling towards them from the Signal Box in the rear, preventing the Signalamn in rear from sending another train towards them until the previous train had cleared the section and passed the signalbox complete, indicated by the trains red tail lamp. The telegraph system was developed to allow two Signalmen to communicate to each other by tapping out a series of bell codes as a message. Developed before the existance of the telephone. Hence why the dangerous practice of Time Interval Working existed when the telegraph/bell-code system between boxes failed.

The block instrument is another key feature. Using similar telegraph technology it indicates at a glance to the signalman at the entry of the section whether his colleague at the exit had accepted a train or whether a train is still passing through the section. Without it, safety would have to rely on memory or on meticulous upkeep of the the train register. Later on, as mentioned above, the block instrument would be electrically locked with the signalling.

 

[thedbdiboy]

This is all bringing back memories of my AB signalling regs training. The one thing that always stood out for me about Absolute Block compared to the various other forms of block signalling was the distinction between the need to 'offer' and 'accept' trains as opposed to the signaller just clearing a train for the section ahead provided it was unoccupied. In very simple terms, under Track Circuit Block one would normally expect green signals when the line ahead is unoccupied, but under AB the default would be red unless specifically cleared for a train. This is, I acknowledge, an oversimplification and all block signalling systems share the same basic principles but is quite a good start point. In that regard Track Circuit Block could be likened to AB with lots of intermediate block sections except that rather than being switched in or out they work automatically.

 

[High Dyke]

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

Indeed. First signal box I signed for had no station limits, but worked Absolute Block to the boxes either side.

 

[Kauntrilla]

A little bit different question. Is anyone able to shed a light how intense and what type of signalling is taught in train driver courses? Thanks

 

[12LDA28C]

A little bit different question. Is anyone able to shed a light how intense and what type of signalling is taught in train driver courses? Thanks

Basically, the stuff that's in the Rule Book, possibly tailored to your particular TOC / FOC depending on the routes that you'll be expected to sign.

 

[Tomnick]

A little bit different question. Is anyone able to shed a light how intense and what type of signalling is taught in train driver courses? Thanks

The main focus is, sensibly, on the signal indications themselves and the implications for drivers. There's relatively little emphasis on what the different modes of signalling (AB, TCB, etc.) actually are. I find, generally, that understanding of the underpinning principles is very poor within the driving grade (at all levels) – not necessarily a huge problem, as drivers don't really need to understand what's going on in the box, but it does allow a few silly ideas to creep in occasionally, and of course an understanding of how it all works does make it easier to learn and retain the rules appertaining to drivers.

 

[Kauntrilla]

Basically, the stuff that's in the Rule Book, possibly tailored to your particular TOC / FOC depending on the routes that you'll be expected to sign.

Thanks mate

The main focus is, sensibly, on the signal indications themselves and the implications for drivers. There's relatively little emphasis on what the different modes of signalling (AB, TCB, etc.) actually are. I find, generally, that understanding of the underpinning principles is very poor within the driving grade (at all levels) – not necessarily a huge problem, as drivers don't really need to understand what's going on in the box, but it does allow a few silly ideas to creep in occasionally, and of course an understanding of how it all works does make it easier to learn and retain the rules appertaining to drivers.

Agree mate. Thanks