Absolute block signalling

Object of the system

The object, or aim, of the absolute block system is in itself defined fairly simply - "to prevent more than one train being in a Block section on the same line at the same time". In absolute block working, a block section (or simply section) is a section of railway line between one signalbox and another - in absolute block, lines are paired, with an up and a down line which run in opposite directions (up being towards the principal terminus - usually London - although the definition of up has little bearing on the actual signalling of trains).

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).

Overview

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.

Block instruments

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 both directions in the section between A and B, and one for trains in both directions in the section between B and C.

Signalling bell (block bell)

The signalling bell, also known as a block bell, is used in conjunction with the block instruments if the bell is not integrated with them. It is a single stroke design and relays the codes from adjacent signal boxes. Each bell has its own distinctive sound to alert the signalman which instrument needs to be attended to.

Example block-bell exchange

Let us consider the process of signalling a train in the up direction (from A to C) past a signal box B. The signal box in rear is A and the signal box in advance is C. The block indicators 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". The "Line Clear" is repeated at box A, and allows the signalman at A to clear, or "pull off", his signals.

At this point, B will not clear any of his signals. Firstly, he cannot clear his starting signal without a "Line Clear" from C. As a result, B will not clear his home signal - he can only clear it when he either has a clear run through (which he does not have without a "Line Clear" from C), or is confident that the train will be able to stop at his starting (or section) signal (this is not done until the train is in view and visibly under control). Finally, his distant will not clear without both his home and starting signals being clear.

As the train passes the starting signal at 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. Then 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". The block section between A and B is now normal and A can offer B another train, if he has one.

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".

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, which controls the exit of an absolute block section, and a section signal which controls the entrance to an absolute block or intermediate block section. Both of these are stop signals, and are capable of showing clear or stop. The extent of the line from the rearmost home signal to the most advanced starting signal controlled from the same signal box is called station limits at that signal box (this does not necessarily refer to a passenger station). A distant signal is also provided some distance from the home signal, which will only show a clear aspect if all stop signals under a signal box's control are clear, and will otherwise show caution - this gives a driver advance warning of a need to stop.

The extent of the line from the most advanced starting (or intermediate block home signal) signal at one signal box to the home signal at the next signal box is called the absolute block section. The absolute block system controls the safe movement of trains in the block section, and no more than one train may ever enter the section at once, other than in exceptional circumstances. Within station limits, the signalman controls the safe movement, and in normal circumstances he can directly see the position of trains there. Usually no communication with other signalmen is needed for movements within station limits.

Some signal boxes are equipped with an intermediate block section, or IBS. This normally takes the place of an old absolute block section, and is commonly found where former absolute block sections and their associated signal boxes have been removed. Essentially an intermediate block section allows two block sections, and therefore two trains, to be on the same line but controlled by the same signal box.

Typically, a signal box with an intermediate block section will have a home signal (and associated distant signal), starting signal and an intermediate block home signal which has its own distant signal. The line from the starting signal to the intermediate block home signal is called the intermediate block home section. The line from the intermediate block home signal to the home signal of the next signal box on the same line in the same direction of travel is the absolute block section. To clear the intermediate block home signal a "line clear" is required from the signal box in advance.

An intermediate block section means that a train can approach the intermediate block home signal while there is a train between the intermediate block home signal and the home signal of the next signal box on the same line in the same direction of travel. Generally, all intermediate block home signals and their respective distants are colour-light signals, normally showing two aspects.

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.

Bell codes

Bell codes are used to communicate with adjacent signal boxes. They can communicate information regarding the type of train being offered, the status of trains within sections or emergency information. A bell code is acknowledged as being understood by repetition.

Nearly all bell codes are preceded by a single stroke on the bell, referred to as Call Attention — the main exception being Train Entering Section. The Is Line Clear? bell signal describes the train, distinguishing between ordinary and express passenger trains, and various categories of goods train. 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. These often vary by location.

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 signalmen in giving trains priority according to their classification, as well as carrying out any special instructions that may apply at a specific location. 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.

Supplemental codes

These codes are supplemented by codes either side, to show the status of the train within the section or the section itself:

Example bell-code exchange

If Box A wishes to pass an ordinary passenger train to Box B the exchange would be as follows:

The train passes the first signal box (in this case A):

The train transits the section complete with tail lamp:

Clearing point

The clearing point is a point usually ¹⁄₄ mile (400 m) in advance of a signal box's home signal.

If, for some reason, the train does not immediately proceed beyond B and remains within the clearing point, the signalman cannot send "Train out of Section" to the signalbox in rear until the clearing point is clear. This is to give some safety margin in the event of a following train misjudging its braking to a stand at the home signal.

Similarly, if the clearing point is obstructed for some other reason, such as shunting, a train cannot be accepted.

The clearing point is usually defined by some obvious marker - at some wayside locations, it may simply be the starting signal in the same direction, whilst at others it may be another signal, or require a track circuit to be clear. Additionally, no two clearing points may overlap - for example, at junctions with conflicting movements, the points should be set so that, should each train fail to stop, they would not collide. Similarly, a full ¹⁄₂ mile (800 m) must be given for trains approaching each other on the same track—¹⁄₄ mile (400 m) for each train.

A further rule regarding clearing points is that they cannot be modified once a train has been accepted, and that they must be locked for passenger train movements.

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 is a physical indication that the train was complete — that is, that it has not become divided in the section, leaving a portion behind (although in reality, almost all modern trains are fully fitted with continuous brakes, and hence both portions of a divided train would come to a stand if separated).

Normally this is done by visual observation of the signalman, although occasionally verbal confirmation, 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.

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 any other information pertaining to the running of trains - such as abnormal events. The train register acts as a memory aid to the signalman, reinforces the systematic working of the block system, and serves as a permanent record of events in case of any mishap or accident.

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 mostly confined to limited areas of the network that carry lighter and slower traffic (there are exceptions, such as the Great Western main line through much of Cornwall). As noted above all Network Rail RUSs point to lines where absolute block signalling is a bar to increased services (both passenger and freight trains). The problem is made worse by rationalisation in the 1960s which closed signal boxes and made some absolute block sections of considerable length.

Some heritage railways also use the absolute block system.

The basic principles of absolute block working were adopted in a number of British Commonwealth nations and are still in use in some areas.