British absolute block signalling

British absolute block signalling

The principle of the British absolute block system of railway signalling is to facilitate the safe operation of a railway by preventing more than one train from occupying a defined section of route at the same time. This system is used on double or multiple lines where use of each line is assigned a direction of travel.

The system was devised and much refined in the second half of the nineteenth century, and successfully managed train control over the vast majority of the British railway system until gradually superseded by more sophisticated systems from 1950.


A railway line under absolute block working is divided into block sections. 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 takes place by bell codes and status indications transmitted over a simple wire circuit between signalmen using a device called a "block instrument".

Block instruments

The block instrument consist of a small cabinet; its front face displays two indicators -- telegraph needles -- and has a commutator handle. 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. The commutator has three positions and each of the two indicators has three positions: Normal (or Line Blocked); Line Clear; and Train On Line.

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.

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.

Some early block instruments had miniature semaphore arms rather than needles.

ections and station limits

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. The extent of the line from the home signal to the section signal controlled from the same signal box is called station limits at that signal box (this does not necessarily refer to a passenger station).

The extent of the line from the section (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. Within station limits, the signalman controls the safe movement, and in normal circumstances he can directly see the position of trains there.

Some signalboxes are equipped with an 'intermediate block section'. This, normally, takes the place of an old absolute block section and is commonly found where former absolute block sections and their associated signalboxes have been removed. Essentially an intermediate block section allows two sections to exist on the same line controlled by the same signalbox.

Typically, a signalbox with an intermediate block section will have a home signal, section signal (and associated distant signal) and an intermediate block home signal which has its own distant signal. The line from the section 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 signalbox 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 signalbox 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 signalbox 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

Bell Codes are the system used to advise the signal boxes next to each other, and are used to advise the next signal box on the line what you are sending to them, or what they are sending you, and the status of transition between controlled sections. A bell code is acknowledged as being understood when it has been repeated back to the signalbox where it originated from.

Nearly all bell codes are preceded by a single stroke on the bell, referred to as "Call Attention". The "Is Line Clear?" bell signal describes the train, distinguishing between ordinary and express passenger trains, and various categories of goods train. There are also a number of bell codes for irregular situations such as emergencies. In some locations, routing information is included in the bell code, for example 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.

Types of train

Train Operating Companies(TOCs) and Freight Operating Companies(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 signallers in giving trains priority according to their classification. 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.

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

The train transits the section:

Clearing point

If for some reason, the train does not immediately proceed beyond B, the signalman must not give "Train Out Of Section" to A, even though the section is clear and the train is wholly within B's station limits. A distance of a quarter mile in advance of B's home signal must be clear before he can give "Train Out Of Section"; this is to give some safety margin in the event of a following train misjudging its braking to a stand at the home signal. The point a quarter mile in advance of the home signal is called the clearing point, and in simple wayside station situations it was usually located at the starting signal.

Tail lamp

The requirement to send "Train Out Of Section" is that train has passed the clearing point complete with tail lamp attached. The tail lamp was a physical indication that the train was complete -- that is, that it had not become divided in the section, leaving a portion behind.

Normally this is done by visual observation from the signaller, although occasionally a tail lamp camera or tail light plunger can be used to verify the train passed the clearing point complete. This is usually done when the train would stop for a long time before passing the signal box, e.g. in a loop or station. Another train cannot be accepted from the box in rear until some form of tail light confirmation has been received.

Train register

A "train register" is used in conjunction with the absolute block system. It is a book in which the signalman must record the time and description of every bell code sent or received, and certain other information. The train register acts as a memory aid to the signalman, and reinforces the systematic working of the block system.

Usage today

The absolute block system enables the safe working of trains between manual signal boxes. As power signalling installations are implemented covering a wide area of control, manual signal boxes are gradually being supplanted, and the absolute block system is now confined to limited areas of the network that have not yet been modernised. One notable example still in existence of absolute block signalling is in the Stockport area including Edgeley Junction. The four boxes, Stockport No.1 & No.2 and Edgeley Junction No.1 & No.2 still work Absolute Block on a primary mainline route, albeit with colour light signals and motorised points. [ [ Edgeley Junction No1 signal box ] ]

The basic principles of absolute block working were adopted in a number of British Commonwealth nations and are likewise still in use in some areas.

ee also

* Rule 55


* [ Information about the absolute block system on]

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