British absolute block signalling
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The principle of the British absolute block system of railway signalling is to prevent more than one train from occupying a block section at the same time. This system is used on double or multiple lines where use of each line is assigned a direction of travel.
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[edit] Overview
A railway line under absolute block working is divided into block sections. A train approaching a section is offered by the signalman at the entering end to his counterpart at the opposite end. If the section is clear, the signalman at the leaving end of the section accepts the train. This communication takes place by bell codes transmitted over a simple wire circuit between signalmen using a device called a block instrument.
[edit] Block instruments
The block instrument at the accepting end of the section is equipped with a commutator lever or knob, and a needle that indicates its state. The corresponding instrument at the other end of the section has a needle that repeats the state shown on the accepting instrument. Some early block instruments had miniature semaphore arms rather than needles.
The original form of block instrument had just two positions: Line Clear and Train On Line. Later instruments had a third indication Line Blocked (or Normal), to indicate that the section was clear but a train had not been accepted.
[edit] Train entering a block section
When the signalman at the leaving end accepts a train, he turns the lever to move the needle from Line Blocked to Line Clear. This action permits the offering signalman to clear his signals to allow the train to enter the section. This is often achieved by interlocking the section signal to the block instrument.
When the train enters the section, the offering signalman sends a "train entering section" bell code, which is acknowledged by his counterpart, who turns his instrument's knob to Train On Line.
[edit] Train leaving a block section
Likewise, when the train leaves the section, the signalman at the leaving end of the section sends a "train out of section" bell code, again acknowledged, and then turns the instrument back to Line Blocked. This is only done providing the signalman sees the lamp attached to the rear of the train. If no lamp is seen then the signalman ahead is signalled to stop the train and inspect it in case it has come apart. Until this is done the signalman keeps the section marked as 'Line Blocked'
[edit] Train register
The absolute block system is accompanied by a train register used to report times and types of codes sent and received, and other pertinent information. The register acts as a memory aid to the signalman, a source of information after an accident, and a way for railway authorities to check (by comparing registers from adjacent sections) that the signalmen were working the lines in a careful, controlled manner as described in the regulations.
[edit] Usage today
While the British absolute block system is being supplanted by Track Circuit Block signalling and centralised control of signalling, it remains in use in isolated pockets. 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.
In some ways the term "absolute" is misleading. There are circumstances when two trains may occupy a block with the permission of the signalman and having been advised to do so. The most common of these is when coupling a locomotive to a train or to join two trains, another example where the block system ceases to be absolute is when a locomotive is sent to rescue a failed train. In station limits, a subsidiary signal may be used that allows a train to proceed into an occupied block. This is mainly used when trains are on separate platforms/sidings within the same block section.
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