Absolute block signalling
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.
Prior to the introduction of block systems, time-intervals were used to ensure that trains were spaced sufficiently apart; typically if five minutes had passed since the first train had departed then a second train was allowed to the proceed; although the driver was warned that there was a train only five minutes ahead.[1]
The electric telegraph provided the ability for signalmen to communicate with each other and provided the basis for the absolute block system.[1] It was devised and much refined in the second half of the 19th century; by 1872 it was used on 44 percent of lines in Britain, rising to 75% by the end of the decade.[1] It successfully managed train control over most of the British railway system until generally superseded by more sophisticated systems from 1950. Absolute block signalling however still constrains capacity on many lines, major and minor (see Network Rail RUS).
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 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.
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.
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. The extent of the line from the home signal to the section signal controlled from the same signal box is called "station limits" at that signal box (this does not necessarily refer to a passenger station).
The extent of the line from the section (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. Within station limits, the signalman controls the safe movement, and in normal circumstances he can directly see the position of trains there.
Some signal boxes are equipped with an "intermediate block section". 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 sections to exist on the same line, controlled by the same signal box.
Typically, a signal box with an intermediate block section will have a home signal, section signal (and associated distant signal) and an intermediate block home signal which has its own distant signal. The line from the section 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 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. A bell code is acknowledged as being understood when it has been repeated back to the signal box where it originated from.
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. There are also a number of bell codes for irregular situations such as emergencies. In some locations, routeing 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 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 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 75 mph, parcels train, nominated (priority) empty passenger trains, autumn railhead treatment train | |
| Class 4 | 3-1-1 | Freight train that can run at up to 75 mph | |
| Class 5 | 2-2-1 | Empty passenger train | |
| Class 6 | 5 | Freight train that can run up to 60 mph | |
| Class 7 | 4-1 | Freight train that can run up to 45 mph | |
| Class 8 | 3-2 | Freight train that can run at, or is timed to run at, 35 mph or less | |
| Class 9 | 1-4 | Class 373 train (Eurostar) (Also used for any other specially authorised train and all trains on the new East London Line). | Previously used for unbraked freight trains, i.e. a train composed of wagons not fitted with automatic brakes |
| Class 9 | 1-4-1 | Empty Class 373 train (Eurostar) | |
| Class 0 | 2-3 | Light locomotive or locomotives (no wagons attached) |
Supplemental codes
These codes are supplemented by codes either side, to show the status of the train within the section or the section itself:
| Bell Code | Meaning | Notes |
|---|---|---|
| 1 | Call Attention | *opening code |
| 2 | Train entering section | Does not require "Call Attention", as the signalman should remember he has a train due. |
| 2 - 1 (2 bells, pause, 1 bell) | Train out of section | |
| 3 - 3 | Blocking back outside Home Signal | |
| 2 - 4 | Blocking back inside Home Signal | |
| 5 - 2 | Release token - Electric Token Block only | |
| 2 - 5 | Token replaced - Electric Token Block only | |
| 5 - 5 - 5 | Opening signal box | |
| 5 - 5 - 7 | Closing of signal box where section signal is locked by the block | |
| 7 - 5 - 5 | Closing of signal box | |
| 6 | Obstruction danger | |
| 7 | Stop and examine | |
| 9 | Train passed without tail lamp - sent to signal box in advance | |
| 4 - 5 | Train passed without tail lamp - sent to signal box in rear | |
| 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.
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 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".
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. 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".
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".
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 | 2 | Train entering section |
| B | 2 | Okay the train is on its way to me |
The train transits the section complete with tail lamp:
| 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 |
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. Additionally, no 2 clearing points may overlap - for example, at junctions with conflicting movements, the points (switches) should be set so that, should each train fail to stop, they would not collide. Similarly, a full 1/2 mile must be given for trains approaching each other on the same track - 1/4 mile for each train.
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.
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. As noted above all Network Rail RUS's 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.