Absolute block signalling

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A block instrument set to its default position of "line blocked"

Absolute block signalling is a British signalling scheme designed to ensure the safe operation of a railway by allowing only one train to occupy a defined section of track (block) at any time.[1] This system is used on double or multiple lines where use of each line is assigned a direction of travel.

A train approaching a section is offered by a signalman to his counterpart at the next signal box. If the section is clear, the latter accepts the train, and the first signalman may clear his signals to give permission for the train to enter the section. This communication traditionally takes place by bell codes and status indications transmitted over a simple wire circuit between signalmen using a device called a block instrument, although some contemporary block working is operated wirelessly. This process is repeated for every block section a train passes through.

Prior to the introduction of block systems, time intervals were used to ensure that trains were spaced sufficiently apart; typically if five minutes had passed since the first train had departed then a second train was allowed to proceed; although the driver was warned that there was a train only five minutes ahead provided that the speed of both trains remains constant.[2]

The electric telegraph provided the ability for signalmen to communicate with each other and provided the basis for the absolute block system.[2] It was devised and much refined in the second half of the 19th century; by 1872 it was used on 44 percent of lines in Britain, rising to 75% by the end of the decade and was made mandatory on passenger-carrying lines in 1889. It successfully managed train control over most of the British railway system until generally superseded by more sophisticated systems from 1950.[2]

In absolute block working, a block section (or simply section) is a section of railway line between one signalbox and another – in absolute block, lines are paired, with an up and a down line which run in opposite directions (up being towards the principal terminus – usually London – although the definition of up has little bearing on the actual signalling of trains).[citation needed]

The absolute block system does not replace the use of any other form of signalling, such as fixed signals, hand signals, or detonators (and, in fact, usually relies on fixed signals).[citation needed]

Block instruments[edit]

British Railways standard block instrument showing Line One's status on top (for the departing train), Line Two's status in the middle (for the approaching train), with the handle at the bottom
Close-up of the commutator, tapper and bell mechanism

The block instrument consists of a small cabinet; its front face displays two indicators — telegraph needles — and has a commutator handle (some early designs of block instruments had miniature semaphore arms instead of needles). The upper indicator shows the state of the forward section, on the line leading away from the signal box. The commutator is used by the signalman to indicate the state of the section approaching his signal box, and the lower indicator repeats the commutator position. All indications are repeated on a similar instrument at the other end of the block section, in the associated signal box. The commutator has three positions and each of the two indicators has three positions: normal (or line blocked), line clear, and train on line. Either integral to the instrument or separately mounted, there is a single-stroke bell and a bell operating device, either a tapper or a plunger.

In a simple double line configuration where the signal boxes are A, B and C in succession, the signal box at B will have two block instruments, one for trains in both directions in the section between A and B, and one for trains in both directions in the section between B and C.

Signalling bell[edit]

A small wooden box with a bell on the cover
Tyer's single stroke signalling bell
The same box with the cover removed, revealing the relay inside
Internal view showing the coil, clapper and relay

The signalling bell, also known as a block bell, is used in conjunction with the block instruments if the bell is not integrated with them. It is a single stroke design and relays the codes from adjacent signal boxes. Each bell has its own distinctive sound to alert the signalman which instrument needs to be attended to.

Example block-bell exchange[edit]

Diagram showing the layout of an example signalling layout.
The location of signals at signalboxes A, B and C. Up is defined as being towards C, and only signals in the Up direction are shown for clarity. Our example train will travel in the Up direction.

Let us consider the process of signalling a train in the up direction (from A to C) past a signal box B. The signal box in rear is A and the signal box in advance is C. The block indicators at B are in the Normal position. The signalman at A "offers" the train to B by sending an "Is Line Clear?" code on the block bell; for example to offer an express passenger train, he sends four beats consecutively; an ordinary passenger train is offered by sending three beats, and after a pause one more beat, usually written as 3-1. If the signalman at B can accept the train safely (see below) he "accepts" the train by repeating the bell signal, and placing the commutator on his block instrument for the section from A to "Line Clear". The "Line Clear" is repeated at box A, and allows the signalman at A to clear, or "pull off", his signals.

At this point, B will not clear any of his signals. Firstly, he cannot clear his starting signal without a "Line Clear" from C. As a result, B will not clear his home signal – he can only clear it when he either has a clear run through (which he does not have without a "Line Clear" from C), or is confident that the train will be able to stop at his starting (or section) signal (this is not done until the train is in view and visibly under control). Finally, his distant will not clear without both his home and starting signals being clear.

As the train passes the starting signal at A, the signalman there sends the "Train Entering Section" signal (2 beats) on the block bell to B, and the signalman at B acknowledges the signal and moves the commutator to "Train On Line". His lower indicator on the block indicator to A repeats the position of the commutator.

B immediately offers the train on to C by sending the "Is Line Clear?" bell signal; if C accepts it he repeats the bell signal and places his block indicator to "Line Clear", which moves the position of the upper needle indicator in B's block instrument to repeat that indication. B may now clear his signals for the train.

After an interval, the train will arrive and pass B; as it does so, B sends "Train Entering Section" on the block bell to C. Then C acknowledges the bell signal and places the block instrument to "Train On Line". As the train passes, he restores his signals to danger, and when the whole of the train passes B complete with tail lamp attached, B sends the "Train Out Of Section" bell signal (2-1) to A and when A acknowledges it, he places his block indicator to "Normal". The block section between A and B is now normal and A can offer B another train, if he has one.

When the train has reached C, the signalman there sends "Train Out Of Section" on the block bell and when B acknowledges it, C places the block indicator to "Normal".

Sections and station limits[edit]

A line of railway is controlled by signalmen in a series of signal boxes. Typically each signal box is equipped with a home signal, which controls the exit of an absolute block section, and a section signal which controls the entrance to an absolute block or intermediate block section. Both of these are stop signals, and are capable of showing clear or stop. The extent of the line from the rearmost home signal to the most advanced starting signal controlled from the same signal box is called station limits at that signal box (this does not necessarily refer to a passenger station). A distant signal is also provided some distance from the home signal, which will only show a clear aspect if all stop signals under a signal box's control are clear, and will otherwise show caution – this gives a driver advance warning of a need to stop.

The extent of the line from the most advanced starting (or intermediate block home signal) signal at one signal box to the home signal at the next signal box is called the absolute block section. The absolute block system controls the safe movement of trains in the block section, and no more than one train may ever enter the section at once, other than in exceptional circumstances. Within station limits, the signalman controls the safe movement, and in normal circumstances he can directly see the position of trains there. Usually no communication with other signalmen is needed for movements within station limits.

Some signal boxes are equipped with an intermediate block section, or IBS. This normally takes the place of an old absolute block section, and is commonly found where former absolute block sections and their associated signal boxes have been removed. Essentially an intermediate block section allows two block sections, and therefore two trains, to be on the same line but controlled by the same signal box.

Typically, a signal box with an intermediate block section will have a home signal (and associated distant signal), starting signal and an intermediate block home signal which has its own distant signal. The line from the starting signal to the intermediate block home signal is called the intermediate block home section. The line from the intermediate block home signal to the home signal of the next signal box on the same line in the same direction of travel is the absolute block section. To clear the intermediate block home signal a "line clear" is required from the signal box in advance.

An intermediate block section means that a train can approach the intermediate block home signal while there is a train between the intermediate block home signal and the home signal of the next signal box on the same line in the same direction of travel. Generally, all intermediate block home signals and their respective distants are colour-light signals, normally showing two aspects.

The signal box towards which a train travels is said to be in advance and the signal box from which it travels is said to be in rear.

Bell codes[edit]

Bell codes are used to communicate with adjacent signal boxes. They can communicate information regarding the type of train being offered, the status of trains within sections or emergency information. A bell code is acknowledged as being understood by repetition.

Nearly all bell codes are preceded by a single stroke on the bell, referred to as Call Attention — the main exception being Train Entering Section. The Is Line Clear? bell signal describes the train, distinguishing between ordinary and express passenger trains, and various categories of goods train. In some locations, routing information is included in the bell code, such an ordinary passenger train to be routed to a branch at the signal box in advance would be offered by the bell code 1-3 instead of 3-1. These often vary by location.

Types of train[edit]

Train operating company (TOCs) and freight operating company (FOCs) services are designated a classification as described below. For passenger trains it is generally dependent on their stopping pattern and for freight trains it is dependent on their maximum permitted speed. Empty coaching stock trains are normally class 5, but can be designated class 3 if they are going to form a Class 1 or 2 service at their destination. It is a generalised guide to assist signalmen in giving trains priority according to their classification, as well as carrying out any special instructions that may apply at a specific location. Class 1 is the highest and subsequent trains in descending order with the exception of class 9 which are officially the equivalent of a class 1.

Type Bell Code Train Notes
Class 1 4 Express passenger train; nominated postal or parcels train; breakdown or snowplough going to clear the line
Class 2 3-1 Ordinary passenger train; breakdown train not going to clear the line; officers special train
Class 3 1-3-1 Freight train capable of running at more than 75 mph; parcels train; nominated (priority) empty passenger trains; autumn railhead treatment train Class 3 Railhead treatment trains signaled using the special 3-4-2 'Is Line Clear' bell code.
Class 4 3-1-1 Freight train that can run at up to 75 mph
Class 5 2-2-1 Empty coaching stock
Class 6 5 Freight train that can run at up to 60 mph
Class 7 4-1 Freight train that can run at up to 45 mph
Class 8 3-2 Freight train that can run at, or is timed to run at, 35 mph or less
Class 9 1-4 Class 373 train (Eurostar); also used for any other specially authorised train and all trains on the new East London Line Previously used for unbraked freight trains, i.e. a train composed of wagons not fitted with continuous brakes
1-4-1 Empty Class 373 train (Eurostar)
Class 0 2-3 Light locomotive or locomotives

Supplemental codes[edit]

These codes are supplemented by codes either side, to show the status of the train within the section or the section itself:

Bell code Meaning Notes
1 Call attention The attention signal is used to confirm that the called box is listening. A single bell is sent to the called box and repeated back to the calling box before each signal is sent.
2 Train entering section Does not require "Call Attention", as the signalman should remember he has a train due.
2 - 1 Train out of section
2 - 2 Engine assisting in rear (known as 'bankers') sent after train entering section—normally to assist freight trains or long passenger trains up steep hills
3 - 3 Blocking back outside Home Signal
2 - 4 Blocking back inside Home Signal
3 - 3 - 2 Shunt into forward section Used where the section signal is locked by the block.
3 - 2 - 2 Shunt into forward section
3 - 3 - 4 Train brought to a stand (stop) only sent on blocking back and shunt moves on a line on which it was traveling in the opposite direction to normal traffic.
5 - 5 Train divided (not applicable if a train is scheduled to be split at a station)
8 Shunt withdrawn
5 - 2 Release token - Electric Token Block only
2 - 5 Token replaced - Electric Token Block only
5 - 5 - 5 Opening signal box
5 - 5 - 7 Closing of signal box where a block switch is provided
7 - 5 - 5 Closing of signal box
6 Obstruction danger Not preceded by "Call Attention" because it is used in emergency. Signalman receiving it must immediately stop, using a red flag, any train travelling towards the signal box from which "Obstruction Danger" was sent; only once he is sure that this has been achieved should he respond.
4 - 5 - 5 Train proceeding without authority in the right direction Sometimes known as "train running away" [3]
2 - 5 - 5 Train proceeding without authority in the wrong direction. Sometimes known as "train running away" [3]
7 Stop and examine train Should be followed, once acknowledged, by a telephone message explaining what is amiss.
9 Train passed without tail lamp – sent to signal box in advance
4 - 5 Train passed without tail lamp – sent to signal box in rear
16 Testing bells and block instruments Performed every time a signal box is opened and every time two signal boxes are connected after an intermediate 'box is switched out.

Example bell-code exchange[edit]

If Box A wishes to pass an ordinary passenger train to Box B the exchange would be as follows:

Box Sends Meaning
A 1 Calling attention
B 1 Attending
A 3-1 Is line clear for a Class 2 train?
B 3-1 Line is clear for a Class 2 train.

The train passes the first signal box (in this case A):

Box Sends Meaning
A 2 Train entering section
B 2 I acknowledge your train entering section.

The train transits the section complete with tail lamp:

Box Sends Meaning
B 1 Calling attention
A 1 Attending
B 2-1 The train has now cleared the section
A 2-1 Acknowledging that the train has cleared the section

See also[edit]


  1. ^ Ellis, Iain (2006). Ellis' British Railway Engineering Encyclopaedia. Lulu.com. p. 6. ISBN 978-1-84728-643-7. 
  2. ^ a b c Faith, Nicholas (2000). "4". Derail: Why Trains Crash. Channel 4 Books. ISBN 0-7522-7165-2. 
  3. ^ a b Rolt, L. T. C. Red For Danger. Pan Books. 

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