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The system was devised and much refined in the second half of the nineteenth century, and successfully managed train control over the vast majority of the British railway system until gradually superseded by more sophisticated systems from 1950. OVERVIEW A railway line under absolute block working is divided into block sections. 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 takes place by Bell Codes and status indications transmitted over a simple wire circuit between signalmen using a device called a ''block instrument''. BLOCK INSTRUMENTS The block instrument consist of a small cabinet; its front face displays two indicators -- telegraph needles -- and has a commutator handle. 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. The commutator has three positions and each of the two indicators has three positions: Normal (or Line Blocked); Line Clear; and Train On Line. 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 both directons in the section between A and B, and one for trains in both directions in the section between B and C. 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. Some early block instruments had miniature semaphore arms rather than needles. SECTIONS AND STATION LIMITS A line of railway is controlled by signalmen in a series of signal boxes. Typically each signal box is equipped with a home signal (controlling the entry of trains to the station area) and a starting signal (controlling the departure of trains from the station area). The extent of the line from the home signal to the starting signal controlled from the same signal box is called the station limits at that signal box (this does not necessarily refer to a passenger station). The extent of the line from the starting signal at one signal box to the home signal at the next signal box is called the section or the '''block section'''. The absolute block system controls the safe movement of trains in the block section. Within station limits, the signalman controls the safe movement, and in normal circumstances he can directly see the position of trains there. The signal box towards which a train travels is said to be in advance and the signal box from which it travels is said to be '''in rear'''. SIGNALLING A TRAIN Let us consider the process of signalling a train past a signal box B. The signal box in rear is A and the signal box in advance is C. The block indictors at B are in the Normal position. The signalman at A "offers" the train to B by sending an "Is Line Clear?" code on the block bell; for example to offer an express passenger train, he sends four beats consecutively; an ordinary passenger train is offered by sending three beats, and after a pause one more beat, usually written as 3-1. If the signalman at B can accept the train safely (see below) he "accepts" the train by repeating the bell signal, and placing the commutator on his block instrument for the section from A to "Line Clear". As the train passes or departs from A, the signalman there sends the "Train Entering Section" signal (2 beats) on the block bell to B, and the signalman at B acknowledges the signal and moves the commutator to "Train On Line". His lower indicator on the block indicator to A repeats the position of the commutator. B immediately offers the train on to C by sending the "Is Line Clear?" bell signal; if C accepts it he repeats the bell signal and places his block indicator to "Line Clear", which moves the position of the upper needle indicator in B's block instrument to repeat that indication. B may now clear his signals for the train. After an interval, the train will arrive and pass B; as it does so B sends "Train Entering Section" on the block bell to C and C acknowledges the bell signal and places the block instrument to "Train On Line". As the train passes, he restores his signals to danger, and when the whole of the train passes B complete with tail lamp attached, B sends the "Train Out Of Section" bell signal (2-1) to A and when A acknowledges it, he places his block indicator to "Normal". When the train has reached C the signalman there sends "Train Out Of Section" on the block bell and when B acknowledges it, C places the block indicator to "Normal". CLEARING POINT If for some reason, the train does not immediately proceed beyond B, the signalman must not give "Train Out Of Section" to A, even though the section is clear and the train is wholly within B's station limits. A distance of a quarter mile in advance of B's home signal must be clear before he can give "Train Out Of Section"; this is to give some safety margin in the event of a following train misjudging its braking to a stand at the home signal. The point a quarter mile in advance of the home signal is called the clearing point, and in simple wayside station situations it was usually located at the starting signal. BELL CODES Nearly all bell codes are preceded by a single stroke on the bell, referred to as "Call Attention". The "Is Line Clear?" bell signal describes the train, distinguishing between ordinary and express passenger trains, and various categories of goods train. There are also a number of bell codes for irregular situations such as emergencies. In some locations, routing information is included in the bell code, for example an ordinary passenger train to be routed to a branch at the signal box in advance would be offerd by the bell code 1-3 instead of 3-1. TAIL LAMP The requirement to send "Train Out Of Section" is that train has passed the clearing point complete with tail lamp attached. The tail lamp was a physical indication that the train was complete -- that is, that it had not become divided in the section, leaving a portion behind. TRAIN REGISTER A ''train register'' is used in conjunction with the absolute block system. It is a book in which the signalman must record the time and description of every bell code sent or received, and certain other information. The train register acts as a memory aid to the signalman, and reinforces the systematic working of the block system. USAGE TODAY The absolute block system enables the safe working of trains between manual signal boxes. As power signalling installations are implemented covering a wide area of control, manual signal boxes are gradually being supplanted, and the absolute block system is now confined to limited areas of the network that have not yet been modernised. The basic principles of absolute block working were adopted in a number of British Commonwealth nations and are likewise still in use in some areas. SEE ALSO REFERENCES AUDIO RECORDING The bell signals referred to can be heard on audio recordings in the Argo Transacord series; the relevant disc is "West of Exeter" and the recordings were made in Tigley signal box and in Exeter West signal box. Both recordings are brief, and the former is not continuous (so that at least one bell signal in the sequence is missing), but they give a flavour of the bell codes in real life. |
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