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Absolute Block

Absolute Block

Commonly abbreviated to AB, Absolute Block is a method of working between two signalboxes. Developed in the mid-19th century, signallers at adjacent boxes communicate with each other to allow trains to enter the section between them using codes sent via bells, and indicators to show the state of the section between them. The block section exists between the last signal controlled by a signalbox (known as the 'Section Signal') and the first signal controlled by the next box along the line (known as the 'Home Signal').

When devised, track circuits did not exist, and most AB sections remain non-track circuited to this day. However, virtually all have had track circuits fitted within the area controlled by a single box, and on the approach to home signals.

After the passage of a train, the section is proved clear by the signaller observing the tail lamp at the rear of the train. Where this is not possible, the traincrew must contact the signaller to confirm that the train is complete. In more recent times, tail lamp cameras have been provided where necessary, or the section track circuited throughout.

AB is generally associated with mechanical signalling and lever frame boxes, though the method of working and type of signalling do not always correspond. For example, there have been instances of a modern workstation being at one end of an AB section. AB is generally used on multi-track lines, though it has been used on uni-directional single track routes.

Since the late 1950s, AB has been steadily replaced with Track Circuit Block working.

AB is represented in numerous SimSig simulations, though primarily TCB is simulated. See individual simulation manuals for details.

A full explanation of AB is beyond the scope of this Wiki. Please see The Signalbox website for a comprehensive explanation.


Last edited by Steamer on 11/04/2021 at 18:24