Absolute block signalling is 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.^[1] Each block section is manually controlled by a signalman, who communicates with the other block sections via telegraph. This system was used on double or multiple lines where use of each line is assigned a direction of travel before the introduction of track circuits, and is still used on lines which lack track circuiting.^{[citation needed]}

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.^[2] This communication traditionally takes place by bell codes and status indications transmitted over a simple telegraph 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. 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.^[3]

Prior to the introduction of block systems, time intervals were used to keep trains sufficiently far 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.^[4] This was insufficient to prevent a train colliding with the rear of one that had stopped unexpectedly.^[5][6]

In absolute block working, a block section (or simply section) is a section of railway line between one signal box and another – in the typical absolute block, lines are paired, with an up – towards London (or in Scotland, Edinburgh) – and a down line in the opposite direction.^{[citation needed]} 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.^[7] A block section is normally the section of track from the most advanced signal controlled by the signal box in rear, and the rearmost signal controlled by the signal box in advance. Because many signal boxes are at stations, the last signal controlled by the rear box is called the starter signal, whether or not there is a station at which trains stop. This signal, and the rearmost signal controlled by the box ahead, are stop signals, which a train should not pass if they are in the on position, showing a red light. The first stop signal controlled by a box is called the home signal.^[8] 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.

Electrical telegraphy was the first practical use of current electricity and was developed in the 1840s and 1850s at the same time as the development of railways. The first commercial electrical telegraph was the Cooke and Wheatstone system. In July 1837 William Fothergill Cooke installed a demonstration system on the Euston to Camden Town section of Robert Stephenson's London and Birmingham Railway for signalling rope-hauling of carriages (as the locomotives could not cope with the steep incline).^[12]

Cooke also put forward the idea of dividing a single line into grand divisions of between 15 and 20 miles (24 and 32 km), each subdivided into stages 2 to 5 miles (3.2 to 8.0 km) long that were to be connected together by telegraph, with instruments that showed the state of each stage.^[13] In 1842 he published these ideas in a book entitled Telegraphic Railways: Or the Single Way Recommended by Safety, Economy, and Efficiency, Under the Safeguard and Control of the Electric Telegraph.^[14] Cooke's ideas were not taken up by the railway companies until the 1850s and 1860s.^[15] When they were developed into a practical system, it provided the ability for signalmen to communicate with each other and provided the basis for the absolute block system.^[5] By 1872 it was used on 44% 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.^[5]

An example is 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.^[2] 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 (if the line is clear up to B's clearing point and will not need to be blocked by another train^{[citation needed]}) 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. In case the line is not clear, B simply does not acknowledge A's "Is Line Clear?", and leaves the commutator in the Normal position.

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, after calling for attention, by sending the "Is Line Clear?" bell signal (repeating the same steps A had done while offering the train to B); 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".^[17]

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.

• Automatic block signaling
• Heritage railway
• Rule 55