page_title: Absolute block signalling
section_title: Block instruments
hierarchical_section_title: Absolute block signalling / Block instruments
caption_attribution_description: English: Close Up of the commutator, Tapper and bell Mechanism. All housed in a compact Bakelite case.
context_page_description: 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 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 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.
context_section_description: The block instrument consists of a small cabinet; its front face displays two indicators — telegraph needles — and has a commutator handle (some early designs of block instruments had miniature semaphore arms instead of needles). The upper indicator shows the state of the forward section, on the line leading away from the signal box. The commutator is used by the signalman to indicate the state of the section approaching his signal box, and the lower indicator repeats the commutator position. All indications are repeated on a similar instrument at the other end of the block section, in the associated signal box. The commutator has three positions and each of the two indicators has three positions: normal (or line blocked), line clear, and train on line. Either integral to the instrument or separately mounted, there is a single-stroke bell and a bell operating device, either a tapper or a plunger.
  In a simple double line configuration where the signal boxes are A, B and C in succession, the signal box at B will have two block instruments, one for trains in down and up direction in the section between AB (trains leaving station B and trains approaching station B), upper and middle part of the first instrument and one for trains in up and down direction in the section between BC (trains leaving station B and trains approaching station B) upper and middle part of the second instrument.
caption: Close-up of the commutator, tapper and bell mechanism
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