What is Permissive Block (or Absolute Permissive Block) Signalling?

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What is Permissive Block (or Absolute Permissive Block) Signalling?

July 16, 2019, 1:05 PM

Normally, in Absolute Block Signalling, the single-line section between points where sidings or loops are provided must be treated as a single block in order to prevent two trains from entering it at the same time. However, this also reduces track utilization in the case of trains following one another in the same direction.

In these cases, a variant of absolute block signalling is used, called Absolute Permissive Block, where for opposing movements (trains in opposite directions), the single-line section is treated as a single block, whereas for following movements (same direction), the single-line section is treated as a sequence of smaller blocks, allowing many trains to be on the single line at the same time.

This is done by having lineside signals at several points along the single-line, which function as normal block signals for following movements; but for opposing movements, all signals for an opposing train turn ‘on’ (‘stop’) for the section of track in front of a train that is already in the single-line section. Permissive block signalling is more often used for freight movements than for passenger trains. Sections with absolute permissive block include Gorakhpur-Chupra (NER) and Bongaigaon-Changsari (NFR).

Source – IFRCA.org

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