Railway Signalling and Operations FAQ: British Block Working

Block Working, Part I

Andrew Waugh

[ Note to Contributors | Copyright & Licensing Agreement ]

To head off some comments, I should point out that 'modern' British block signalling has a variety of mechanical and electrical devices to ensure accuracy...

1. Simple Block Working

Assume that an ordinary passenger train is at Signal Box 'A' and is to run to Signal Box 'C'. The double line between 'A' and 'C' is divided into two absolute block sections by the Signal Box 'B'; the sections being 'A' to 'B' and 'B' to 'C'. Only one train may be in each absolute block section at one time.

The block sections are controlled by block instruments. One instrument is located at each end of the section in the Signal Box. The two instruments at each end of the section are electrically connected.

For the purposes of this article, we will assume that the instruments are similar to standard British Railways three position three wire block instruments.

                  Block            Block          Block
               Instrument       Instruments     Instrument
                  at A             at B           at C
             (for section)    (for sections)   (for section)
                (A to B)     (A to B)(B to C)   (B to C)
                 +-----+      +-----+ +-----+    +-----+
    Train Going  |+---+|      |+---+| |+---+|    |+---+|
    Indicator -->|| / ||      || | || || | ||    || | ||
                 |+---+|      |+---+| |+---+|    |+---+|
    Train Coming |+---+|      |+---+| |+---+|    |+---+|
    Indicator -->|| | ||<---->|| / || || | ||<-->|| | ||
                 |+---+|      |+---+| |+---+|    |+---+|
    Switch ----->|  I  |      |  /  | |  I  |    |  I  |
    Bell Key --->|  o  |      |  o  | |  o  |    |  o  |
                 +-----+      +-----+ +-----+    +-----+

               'A'                  'B'                 'C'

Figure 1. The line from Signalboxes 'A' to 'C' and the four block instruments. Notice that the Signalman at 'B' has turned the switch on the 'A' to 'B' instrument to the 'Line Clear' position causing the 'Train Coming' Indicator on this instrument and the 'Train Going' Indicator on the instrument at 'A' to deflect.

Each block instrument is identical and consists of two three position indicators, a three position switch, a bell key, and a bell. The three positions of both indicators and the switch are labelled "Line Clear", "Normal", and "Train on Line". The indicators are almost invariably galvometers with the "Normal" position indicated by the galvometer needle in a vertical position. The other two positions are indicated by the needle sloping to the left or right. The top indicator of a block instrument controls the line used by trains leaving this Signalbox for the other end of the section. In this article, this indicator will be known as the "Train Going To" indicator. The bottom indicator controls the line used by trains approaching the Signalbox. This indicator will be known as the "Train Coming From" indicator.

The indicators and switch are so connected such that the switch controls the bottom indicator at the local Signalbox and the top indicator of the block instrument at the other end of the section. For example, if the switch was turned to "Line Clear", bottom indicator in the same block instrument would also deflect to "Line Clear" and, at the same time, so would the upper indicator in the block instrument at the other end. It is important to note that the indicator for the line leaving the Signalbox is always controlled by the switch in the block instrument at the other end of the section. Consequently, a Signalman always controls trains approaching his Signalbox.

When the bell key is depressed the bell in the other instrument will sound. Single stroke bells are used so that the bell will only sound once, no matter how long the bell key is held down. Different bell signals are sent by sending different patterns of bell strokes.

1.1. The Basic Sequence

Assuming the block indicator at 'A' shows the normal position, the Signalman at 'A' may attempt to obtain "Line Clear" for the train at 'A' to proceed to 'B'.

The Signalman at 'A' sends "Call Attention" (1 beat of the bell) to 'B'. The Signalman at 'B' will acknowledge the "Call Attention" by repeating the bell signal back to 'A'. This shows that the Signalman at 'B' is waiting at his block instrument.

The Signalman at 'A' then asks for 'Line Clear' from 'B'. To do this, he sends the prescribed "Is Line Clear?" signal. Assuming that the train was a stopping passenger train the signal would be "Is Line Clear for Ordinary Passenger Train?" signal (3 pause 1 beats). If the line is clear at 'B', the Signalman there will give "Line Clear" by repeating the bell signal back to 'A' (3 pause 1) and turning the switch of his block instrument to the "Line Clear" position. This will cause the "Train Coming From" indicator on his block instrument to show "Line Clear". Simultaneously, the "Train Going To" needle on the instrument at 'A' will also move to show "Line Clear". This is a visual reminder to both signalmen that "Line Clear" has been asked and given.

Having got "Line Clear" from 'B', the Signalman at 'A' can now clear the signals for the passenger train to depart. As the train leaves, the Signalman at 'A' sends "Train Entering Section" (2 bells) to 'B'. The Signalman at 'B' acknowledges this signal by repeating it and turns the switch on the block instrument from "Line Clear" to "Train on Line". This would make the 'Train coming from' needle at 'B' and the 'Train going to' needle at 'A' point to "Train on Line".

Having received "Train Entering Section" from 'A', the Signalman at 'B' now gets "Line Clear" from 'C' for the train. This is obtained in exactly the same way 'A' obtained "Line Clear" from 'B'; first the "Call Attention" and then the "Is Line Clear?" signal are sent and acknowledged. When the Signalman at 'B' obtains "Line Clear" from 'C' he may clear the signals for the approaching train to pass through the station.

As the passenger train passes through 'B', the Signalman watches its passage to make sure that there is nothing amiss. He also checks that the tail lamp is displayed on the rear of last vehicle on the train. If the lamp is present then the Signalman at 'B' knows that the train has arrived complete at 'B' and no vehicles have been left behind in the section.

The Signalman at 'B' then sends the "Call Attention" bell signal to 'A'. When this is acknoledged, the Signalman at 'B' can send the "Train Out Of Section" (2 pause 1) to 'A' and restore the commutor of the block instrument to the vertical, "Line Blocked" position. When the Signalman at 'A' repeats this signal, the section between 'A' and 'B' is considered clear and another train can be offered by 'A'.

Having described the basic procedure for passing trains from one Signalbox to another, I will now elaborate on some of the complications I've glossed over.

1.2. Classes of Trains

The block system has different "Is Line Clear" bell signals for different types of trains. The different bell signals function as a simple train describer and inform the signalman at 'B' the type of the approaching train. Table 1 is the list from the 1912 Great Central Railway Rulebook and is probably fairly close to the "standard" Railway Clearing House Rulebook at the time.

As can be seen, the main distinction was between passenger trains (bell codes with 4 beats) and goods trains (bell codes with 5 beats)[1].

Table 1. "Is Line Clear?" Signals used on the Great Central Railway, 1913.

Is Line Clear for Express Passenger Train, or Breakdown Van Train going to clear the Line, or Light Engine going to assist disabled Train? 4
Is Line Clear for ordinary Passenger Train, or Breakdown Van Train not going to clear the Line? 3 pause 1
Is Line Clear for Branch Passenger Train? 1 pause 3
Is Line Clear for Fish, Meat, Fruit, Horse, Cattle or Perishable Train, composed of Coaching Stock? 5
Is Line Clear for Empty Coaching Stock Train? 2 pause 2 pause 1
Is Line Clear for Fish, Meat, or Fruit Train composed of Goods Stock, Express Cattle, or Express Goods Train, Class A? 3 pause 2
Is Line Clear for Express Cattle, or Express Goods Train, Class B? 1 pause 4
Is Line Clear for Light Engine, or Light Engines coupled together or Engine and Brake? 2 pause 3
Is Line Clear for Through Goods, Mineral, or Ballast Train? 4 pause 1
Is Line Clear for Ordinary Goods, or Mineral Train stopping at intermediate Stations? 3
Is Line Clear for Branch Goods Train? 1 pause 2
Is Line Clear for Ballast Train requiring to stop in Section? 1 pause 2 pause 2
Is Line Clear for Platelayers' Lorry requiring to pass through tunnel? 2 pause 1 pause 2

The different classes of trains have different priority and this is reflected in the ordering in the table of bell signals. Priority decreases down the table so, for example, in 1912 an Express Passenger train had the highest priority and a light engine was more important than a through goods.

A Signalman uses the different bell codes to decide what to do with an approaching train. For example, a through goods (4-1) would be side-tracked to allow a Class A Fish Train (3-2) to pass it.

In addition to priority, the different classes indicated the speed of the approaching train. A Cattle Train composed of Coaching Stock (5), for example, would be composed of vehicles which could run at passenger train speeds (i.e. fully equipped with automatic brakes and with axle boxes designed for high speed running). The speed of such a train would be far higher than that of a through mineral train (4-1) which would be a composed of unbraked loose coupled waggons with grease axle boxes.

A Signalman would know how long each class of train would take to traverse the block sections and uses this information to decide what to do with a train. For example, say the Signalman at 'B' had just accepted a through goods train (4-1) from 'A'. He knows that it will take 15 minutes to traverse the section from 'A' to 'B'. Once at 'B' he can either shunt the goods clear of the main line (which will take 10 minutes) or he can let it run to 'C' (which will take 18 minutes). The Signalman also knows that the goods is being followed by an ordinary passenger train. The passenger train will take 6 minutes to run from 'A' to 'B' and 8 minutes to run from 'B' to 'C'. If the passenger train is due at 'A' in 25 minutes, the Signalman at 'B' should shunt the goods at 'B' (15 minutes running time to 'B' plus 10 minutes to shunt it clear) so as to provide a clear run for the passenger train. If he let the goods run to 'C' then the passenger train would be blocked for 12 minutes at 'B' as the goods train would take 28 minutes to clear the 'B' to 'C' section (18 minutes to run from 'B' to 'C' and then 10 minutes to shunt clear of the main line). If the passenger train was 37 minutes behind the goods, then the Signalman at 'B' could let the goods run to 'C' as their would be sufficient time for the Signalman at 'C' to shunt the goods there without blocking the passenger train. If the passenger train is less than 25 minutes behind the goods then the Signalman at 'A' has stuffed up! He should have shunted the goods at 'A' and not let it run to 'B'.

As train speeds increased over the century, additional classes were added. The speed at which goods trains could run largely depended on the brake power of the train. With the gradual increase in the number of 'fitted' waggons (i.e. waggons equipped with automatic brake gear) it became possible to timetable trains which would contain a minimum percentage of automatically braked waggons. These trains could run at a higher speed than the unbraked goods. As a result new classes were added for these trains.

In the last twenty years, the use of unbraked waggons has (almost?) ceased; consequently some of the train classes have been deleted. New classes have been added, however, with the general increase in train speeds.

Another use of the different classifications was to enable the Signalman at 'B' to route trains appropriately. A goods train might be routed into a goods running loop or a goods yard. A stopping passenger train might be routed into a platform road, whilst an express could be run through a non platform road.

A final use of the different classes was to enable the Signalman at the box in advance to accept trains under the 'Section clear, but station or junction blocked' signal. This signal will be described in a later section.

Not all railway companies used the standard "Is Line Clear" bell codes. The Southern Railways used quite a different set, reflecting their different traffic mix (large numbers of passenger trains following complicated routes and relatively small numbers of goods trains). Railway Companys also added additional codes to the standard code to represent trains used on their lines. The GWR, for example, used "Is Line Clear for Rail Motor-Car, Auto Train, or Streamline Railcar?" (3 pause 1 pause 3). In addition, at many complicated locations it was common to provide special bell codes to further differentiate between trains of a particular class. The bell codes at Plymouth, for example, were described[2] as

"[...] GW expresses were 4 beats, SR 2-2-1-4. Stopping passengers 3-1 and 2-2-1-3 respectively. Tavis- tock branch trains were 1-3 and railcars 3-1-3-1-3. Railcars to Plymton and Laira were 3-1-3 and to Saltash 4-3-1-3. Prior to 1941 when Millbay station closed, Down express passenger trains to Cornwall were 4-2-4 and stopping passengers 4-3-1. SR rail- cars between Friary and St. Budeaux ran as 2-3-1-3 until the service ceased."

Larry Crosier described these as not too bad - the extra bell codes for goods trains were worse and those for light engines were worst of all!

1.3. Giving Line Clear

Before the Signalman at 'B' can give "Line Clear" to 'A' for a train to approach, the line has to be clear for 440 yards (1/4 mile) beyond the first home signal at 'B'.

This distance, the overlap, provides a margin of error for the Driver of the approaching train. If the Driver can not stop the train precisely at the home signal, perhaps because the rails approaching the signal were unusually slippery, or because the Driver misjudged his approach, this ensured that the train would not run into an obstruction immediately beyond the home signal. The Driver is, however, still expected to stop at the signal. Until recently most goods trains did not have continuous brakes and passenger trains were equipped with the automatic vacuum brake which is less powerful (and is slower acting) than the Westinghouse air brake.

In addition, the overlap allows trains to travel at a slightly higher overall speed. If a Driver is approaching a station and he knows there is no overlap beyond the first home signal, he must approach the station extremely cautiously. He must assume the worst possible conditions of slippery rails, weak brakes, and an obstruction immediately beyond the signal. This combination rarely happens and, in consequence, the Driver has to reduce speed unnecessarily. As unbraked trains were slow to slow down (or speed up!), this improvement in speed could be most important.

The overlap usually has to be the line on which the train is to run; this is an important reason for including the routing information in the "Is Line Clear" signal. Before granting "Line Clear", the signalman at 'B' has to set the route for the offered train and make sure that there are no trains on the route in the overlap or fouling the overlap. Where possible, points are set to provide to provide traps to prevent other trains from fouling the overlap.

Having granted "Line Clear", the Signalman has to maintain the route and prevent trains from infringing on the overlap. The overlap had to be maintained until the train has: i) passed through the overlap; ii) come to a stand at the home signal; or iii) been cancelled.

Maintaining the overlap can cause considerable inconvenience in the operation of station at 'B'. For example, 'B' might have a layout like that show in Figure 2.

                  Goods Yard
     'A'          ---------        'B'                  'C'
           1|-O  /            |____________|
                               ------------   O-|2
                              |            |

Figure 2. Overlap problems at a station. '1' is the home signal for trains from 'A' and '2' is the home signal for trains from 'C'.

The Signalman at 'B' has given "Line Clear" to 'A' for a train. Train CCCCC now cannot shunt the Goods Siding as to do so would require it to cross the line from 'A' within the overlap. If 'B' is offered a train from 'C', he cannot accept it as Train DDDD is standing at the station platform in the overlap.

Another problem occurs at a double line junction, such as shown in Figure 3.

      'A'                     'B'                 'D'
                                /    ------------------<
                               /   /   O-|2
                         1|-O /   /

Figure 3. Overlap problems at a double line junction.

The Signalman at 'B' could not accept trains from both 'C' and 'D' at the same time as these signals shared a common overlap in front of signals 2 and 3.

Further, if a train from 'D' was accepted the points for trains from 'A' ahead of home signal 1 would have to be set for the line to 'D' (as otherwise any train or vehicles from 'A' - accepted by 'B' or not - might foul the overlap). A train from 'A' could be accepted at the same time as a train from 'D', the overlap for the train from 'A' being along the line to 'D'. If the train from 'A' was actually for 'C', it would have to be brought to a stand at signal 2 before the points could be reset for the train to continue (after the train from 'D' had passed through of course!)

1.4. Refusing Line Clear

If, for some reason, the line is not clear at 'B' for the train to approach (usually because the overlap is not clear), the Signalman would refuse to give "Line Clear" by ignoring the bell signal. The Signalman at 'A' can understand this as a refusal because the Signalman at 'B' has acknowledged the "Call Attention" signal. The Signalman at 'A' should now periodically poll the Signalman at 'B' by going through the "Call Attention", "Is Line Clear?" cycle until the train is accepted by 'B'. In practice, however, the Signalman at 'A' would telephone the 'B' to ask why "Line Clear" had been refused and a time when he should try again.

1.5. Forwarding Line Clear to 'C'

Unless special instructions were in force, the receiving of "Train Entering Section" from 'A' would be the signal for 'B' to offer the train to 'C'.

To prevent delays to trains, 'A' and 'B' had to be far enough apart that 'B' could get "Line Clear" from 'C' and clear his signals before the approaching train sighted the distant signal. If the distant signal hadn't been cleared by the time the approaching train passed it, the train would have to commence slowing down in preparation for a possible stop at the home signal. This check could waste a considerable amount of time.

To avoid this, if block posts were 'close', 'B' would offer the train to 'C' as soon as he had accepted the train from 'A'. The sections considered 'close' would be defined in special instructions.

To simplify the working of a row of such short sections, the GWR introduced the "Train Approaching" signal (1-2-1). Imagine 'A', 'B', 'C', 'D' (etc) were all close together. Immediately upon giving "Line Clear" to 'A', 'B' would get line from 'C' and 'C' from 'D'. The Signalman at 'D', however, would hold the "Line Clear". As the train left 'A', "Train Entering Section" would be sent in the usual way. After acknowledging this signal, 'B' would send "Train Approaching" to 'C' who would repeat it to 'D'. A minute or so after the train had left 'A', this fact would be known by the signalman at 'D' who would then ask "Line Clear" from 'E'.

Normally the minimum distance apart that block posts could be situated was 440 yards (the overlap). In some complicated station yards Signalboxes were situated even closer than this. Assume that Signalbox 'C' was only 200 yards from Signalbox 'B'. Before the Signalman at 'B' could give "Line Clear" to 'A', he would have to obtain "Line Clear" from 'C'. Effectively 'A' would get "Line Clear" from 'C'. This procedure would also be adopted where it was not desired to stop a train at 'B'. Possible reasons include a heavy rising gradient at 'B' or a junction that the train would foul while waiting "Line Clear" from 'C'.

1.6. Giving Train Out of Section

The "Train Out of Section" signal can be sent when the train has passed 1/4 of a mile beyond the first home signal (i.e. beyond the overlap) and when the Signalman knows that the train has arrived complete.

Normally, the Signalman knows that train has arrived complete by observing the red tail lamp on the last vehicle as the rear of the train passes the Signalbox. Sometimes the train is shunted clear of the main line before the rear of the train passes the Signalbox, or, alternatively, the train may come to a stand (at a platform, for example) clear of the overlap but before passing the Signalbox. In these cases the Guard or Shunter may indicate to the Signalman that the train has arrived complete so that the section can be cleared.

"Train out of Section" could also be given before the rear of the train had passed out the overlap if the Signalman could alter a set of points behind the train to provide a different overlap. Imagine a station with a through main line and a platform loop. A stopping passenger train would be let into the loop to stand at the platform. The Signalman could reset the points at the start of the loop to lie for the through main line and give "Train out of Section" to the box in the rear.

Originally, the Signalman did not have to wait for the rear of the train to clear the overlap to give "Train Out of Section". The train could be cleared off as soon as the train had passed the first home signal (provided, of course, that the Signalman knew that the train had arrived complete!). The standard code had changed to require the overlap to be clear by the late 1930's. This probably parallels the decline in the use of the "Section clear, but station or junction blocked" signal.

It was not until the BR era that sending the "Train out of Section" had to be preceded by the "Call Attention" signal. In fact, on the GWR the "Train out of Section" did not even have to be acknowledged by the Signalman in the rear.

1.7. The Train Register Book

The Signalmen at all block stations keep a permanent record of all bell signals sent and received[3] in the Train Register. At busy Signalboxes a Block boy would be provided to record the information and leave the Signalman to concentrate on working the trains. This job was an excellant way to learn how to become a Signalman!

The Train Register had at least three uses. It's main use was as an aid to the Signalman. It was very easy, particularly at late at night, to get confused about the movements of trains. A Signalman might simply forget to get "Line Clear" for a train (possibly because he has confused it with the previous train) and give line for a following train. These sort of errors can be caught if the Train Register is kept up to date.

The second use was as a check on the Signalmen. Signalmen were not closely supervised and a few (a very few) Signalmen took advantage of this fact and got very laxidasical about the Block rules. Periodically, usually fortnightly, the Train Registers would be collected and a random sample selected checked to see that the Signalmen were following the rules. The books would also be compared with the Registers kept by the adjacent boxes. Discrepencies were noted and the offender would receive a "please explain"!

The final use was as a "black box" recorder to attempt to reconstruct what went wrong in the case of an accident.


[1] Except for ordinary goods trains which were 3 beats. This was apparently at the insistance of the Midland Railway Company, who felt that as this was the most common train on their system it should be the shortest bell code.

[2] Larry Crosier, Learning the District - Plymouth No 2, SRS Newsletter, No 72 page 9.

[3] Except call attention.

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Block Working in Britain

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