British absolute block signalling
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The principle of the British absolute block system of railway signalling is to facilitate the safe operation of a railway by preventing more than one train from occupying a defined section of route at the same time. This system is used on double or multiple lines where use of each line is assigned a direction of travel.
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century, and successfully managed train control over the vast majority of the British railway system until gradually superseded by more sophisticated systems from 1950.
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[edit] 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.
[edit] 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 directions 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.
[edit] 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.
[edit] Bell codes
Bell Codes are the system used to advise the signal boxes next to each other, and are used to advise the next signal box on the line what you are sending to them, or what they are sending you, and the status of transition between controlled sections.
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.
[edit] Types of train
Train Operating Companies(TOCs) and Freight Operating Companies(FOCs) services are designated a classification as described below. For passenger trains it is generally dependent on their stopping pattern and for freight trains it is dependent on their maximum permitted speed. Empty coaching stock trains are normally class 5, but can be designated class 3 if they are going to form a Class 1 or 2 service at their destination. It is a generalised guide to assist signallers in giving trains priority according to their classification. Class 1 is the highest and subsequent trains in descending order with the exception of class 9 which are officially the equivalent of a class 1.
| Type | Bell Code | Train | Notes |
|---|---|---|---|
| Class 1 | 4 | Express passenger train, nominated postal or parcels train, breakdown or snowplough going to clear the line or returning from there | |
| Class 2 | 3-1 | Ordinary passenger train, breakdown train not going to clear the line, officers special train | |
| Class 3 | 1-3-1 | Freight train capable of running at more than 75mph, parcels train, nominatied (priority) empty passenger trains, autumn railhead treatment train | |
| Class 4 | 3-1-1 | Freight train that can run at up to 75mph | |
| Class 5 | 2-2-1 | Empty passenger train | |
| Class 6 | 5 | Freight train that can run up to 60mph | |
| Class 7 | 4-1 | Freight train that can run up to 45mph | |
| Class 8 | 3-2 | Freight train that can run at, or is timed to run at, 35mph or less | |
| Class 9 | 1-4 | Class 373 train (Eurostar) | |
| Class 9 | 1-4-1 | Empty class 373 train (Eurostar) | |
| Class 0 | 2-3 | Light locomotive or locomotives (no wagons attached) |
[edit] Supplemental codes
These codes are supplemented by codes either side, to show the status of the train within the section or the section its self:
| Bell Code | Meaning | Notes |
|---|---|---|
| 1 | Attention | *opening code |
| 2 | Train now entering section on its way to you | |
| 2 - 1 (2 bells, pause, 1 bell) | Train (from you is) now clear of section | |
| 3 - 3 | Blocking back (outside Home Signal) | |
| 2 - 4 | Bocking back (inside Home Signal) | |
| 5 - 2 | Release token | |
| 2 - 5 | Token replaced | |
| 5 - 5 - 5 | Opening signal box | |
| 6 | Obstruction danger | |
| 9 | Train heading for you had no tail light on the rear when it passed my box | |
| 16 | Testing bells and instruments |
- The Attention signal is used to confirm that the called box is listening. A single bell is sent to the called box and repeated back to the calling box before each signal is sent.
[edit] 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".
[edit] Example bell code exchange
If Box A wishes to pass an ordinary passenger train to Box B the exchange would be as follows:
| Box | Sends | Meaning |
|---|---|---|
| A | 1 | Calling attention |
| B | 1 | I am listening |
| A | 3-1 | I have a passenger train coming your way |
| B | 3-1 | Okay send me the train |
The train passes the first signal box (in this case A):
| Box | Sends | Meaning |
|---|---|---|
| A | 1 | Calling attention |
| B | 1 | I am listening |
| A | 2 | Train entering section |
| B | 2 | Okay the train is on its way to me |
The train transits the section:
| Box | Sends | Meaning |
|---|---|---|
| B | 1 | Calling attention |
| A | 1 | I am listening |
| B | 2-1 | The train has now cleared the section |
| A | 2-1 | Okay the train has cleared the section |
[edit] 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.
[edit] 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.
Normally this is done by visual observation from the signaller, although occasionally a tail lamp camera or tail light plunger can be used to verify the train passed the clearing point complete. This is usually done when the train would stop for a long time before passing the signal box, e.g. in a loop or station. Another train cannot be accepted from the box in rear until some form of tail light confirmation has been received.
[edit] 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.
[edit] 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. One notable example still in existence of absolute block signalling is in the Stockport area including Edgeley Junction. The four boxes, Stockport No.1 & No.2 and Edgeley Junction No.1 & No.2 still work Absolute Block on a primary mainline route, albeit with colour light signals and motorised points.[1]
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.
