Absolute block signalling

rdf:langString Absolute block signalling

rdf:langString A small wooden box with a bell on the cover

rdf:langString The same box with the cover removed, revealing the relay inside

rdf:langString Internal view showing the coil, clapper and relay

rdf:langString Tyer's single stroke signalling bell

rdf:langString horizontal

rdf:langString British Railways standard block instrument. At the bottom is the tapper, bell and the commutator switch that is used to set the status of the block . Above that is the indicator of the block’s status which is also seen in an adjacent signal box. At the top is a repeater display of the status of the adjacent block.

rdf:langString Block Instruments-11.JPG

rdf:langString Block Instruments-2.JPG

rdf:langString Block Instruments-8.JPG

rdf:langString Block Instruments-9.JPG

rdf:langString 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 a time. This system is used on double or multiple lines where use of each line is assigned a direction of travel. 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 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. 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. This was insufficient to prevent a train colliding with the rear of one that had stopped unexpectedly.