Absolute block

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. This system is used on double or multiple lines where use of each line is assigned a direction of travel.

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.

The electric telegraph provided the ability for signalmen to communicate with each other and provided the basis for the absolute block system. 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 It successfully managed train control over most of the British railway system until generally superseded by more sophisticated systems from 1950.

The object, or aim, of the absolute block system is in itself defined fairly simply - "to prevent more than one train being in a Block section on the same line at the same time". 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).

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).

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.

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Wikipedia