Absolute block signalling
[citation needed] A train approaching a section is offered by a signalman to his counterpart at the next signal box.
[2] 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.
[4] This was insufficient to prevent a train colliding with the rear of one that had stopped unexpectedly, as happened at the Clayton Tunnel rail crash.
[11] Within station limits, the signalman controls the safe movement, and in normal circumstances he can directly see the position of trains there.
The first commercial electrical telegraph was the Cooke and Wheatstone system.
In July 1837 William Fothergill Cooke installed a demonstration system on the Euston to Camden Town section of Robert Stephenson's London and Birmingham Railway for signalling rope-hauling of carriages (as the locomotives could not cope with the steep incline).
[12] Cooke also put forward the idea of dividing a single line into grand divisions of between 15 and 20 miles (24 and 32 km), each subdivided into stages 2 to 5 miles (3.2 to 8.0 km) long that were to be connected together by telegraph, with instruments that showed the state of each stage.
[citation needed] Originally the different displays and commutator handle were in a variety of cabinets.
The standard British Railways block instrument brought them together in a single small cabinet; its front face displaying two indicators, a commutator handle, a bell and a tapper.
The upper indicator shows the state of the forward block – along the line leading away from the signal box.
The upper display is a repeater from the signal box of the block ahead.
[citation needed] Each bell has its own distinctive sound so that the signalman knows which box is communicating with him.
Each communication starts with a single strike of the bell meaning “Call attention”.
The recipient signalman then shows that he has received the message by repeating it back to the sender.
All subsequent bell messages are acknowledged promptly by repeating back to the sender – with the single exception of six strikes which indicate “Obstruction danger” which is not echoed back until all relevant signals have been set to “Stop”.
B immediately offers the train on to C, after calling for attention, by sending the "Is Line Clear?"
In some locations, routing information is included in the bell code, such an ordinary passenger train to be routed to a branch at the signal box in advance would be offered by the bell code 1-3 instead of 3-1.
[18] It is a generalized guide intended to assist signalling staff in prioritizing trains according to their importance as well as ensuring that any special instructions that may apply at a specific location are carried out.
