Signal boxes communicate using single-stroke bells, and a standard code has been developed for these bells over time. This page describes these codes; it is based on the Railtrack rule book as of August 1996, but also lists some codes that only appear in Special Instructions and some historical codes that are no longer used. In some cases the Southern Region used to do things differently; these are described at the end. Note that it is not my intention to describe every code ever used, but to list current practice and some interesting historical bits.
To make it easier to understand, the descriptions of codes use an imaginary section of line with four signal boxes:
Unless explicitly stated otherwise, codes are being sent from Blackbury to Cliffside in association with trains moving on the Down line.
These descriptions assume the reader understands terms such as block instrument and home signal and the basic operation of the block system. A number of rules require the signalman to move the block instrument in various ways, but these are not all described.
Each pair of signal boxes control the section of line between them and communicate using single stroke bells and a pair of block instruments. The same bell is used for both the Up and Down lines. However, if there are two tracks in the same direction a bell and block instrument is provided for each. So there are two bell circuits between Anytown and Blackbury, one for the two main lines and one for the Down Goods (if there was also an Up Goods, this would use the same bell as the Down Goods), and Blackbury has three bells to work with.
In general, a code is acknowledged by repeating it back to the sender. If the recipient cannot agree with the message in the code, he simply ignores it and does not reply. Since most codes are preceded by Call Attention, the sender knows that the recipient is present and paying attention.
The codes are divided into 7 major groups:
All codes must be preceded by Call Attention except for those marked *. Names in italics show codes that are obsolete while those in bold italics are only used in Special Instructions.
The following tables show the codes sorted either by code pattern or by length (in number of bell strokes). "ILC" is short for "Is Line Clear for a".
|Sorted by code||Sorted by length|
The Call Attention bell code is used at the start of most sequences of codes to allow the recipient to be ready for a message, and for the sender to be certain that the recipient is there. If no response is sent the sender will try again every minute or so.
Call Attention is not sent before an acknowledgement, nor before those codes marked with a *. When a sequence of codes is being sent as part of a single operation, it is not necessary to use Call Attention before each one , but only once at the start of the sequence.
|4-4-4||The Royal Train|
|4||Class 1 train (express passenger)|
|4-2||Class 1 electric train|
|4-4||Class 1 train with special timings|
|3-1||Class 2 train (other passenger)|
|3-1-2||Class 2 electric train|
|1-3||Class 2 train for branch|
|1-2||Class 3 to 9 train for branch|
|1-3-1||Class 3 train (express parcels able to run at 90mph)|
|3-2-5||Class 4 (Freightliner) train|
|3-1-1||Class 4 train (express freight or parcels able to run at 75mph)|
|2-2-1||Class 5 train (empty coaching stock)|
|2-2-1-2||Class 5 electric train|
|5||Class 6 train (fully braked Company, parcels, or milk train)|
|4-1||Class 7 train (fully braked freight train)|
|3||Class 7 train stopping in section|
|3-2||Class 8 train (partially braked freight train)|
|1-2-2||Class 9(a) train (freight train with a few braked vehicles)||
|1-4||Class 9(b) train (unbraked freight train)|
|2-3||Class 0 train (one or more light locomotives)|
|1-1-3||Light locomotive with one or two brake vans|
|2-2-3||Train required to stop in section|
|2-6-3||Train requiring the opposite or an adjoining line to be blocked and kept clear|
|2-6-2||Train which cannot be allowed to pass trains on another line signalled 2-6-2 or 2-6-3|
|2-6-1||Train which can pass trains on another line signalled 2-6-1 or 2-6-2 but will be signalled 2-6-2 or 2-6-3 at some later point in its journey|
|2-1-6||Train which carries speed or route restrictions only and can pass a train on another line signalled 2-6-1 or 2-6-2|
|1-2-6||Line to be blocked for passage of train conveying out of gauge on another line|
|2-3-3||Train to run in the wrong direction along a double line (the Up line in this case)|
All of these codes have the same meaning: Blackbury wishes to send a train to Cliffside and is asking permission to do so. If it is safe for the train - that is, the line is clear up to the first home signal at Cliffside and for a further 440 yards (400 metres) - Cliffside will accept the train by repeating the code back and then moving the Block Instrument to Line Clear. If it is not safe, Cliffside simply ignores the bell; Blackbury will try again every couple of minutes.
Note how the main codes for passenger trains use four bells while those for goods trains use five, with the pause shifting forward as we progress from class 1 to 2 or from 6 to 9. Of course, too many codes are needed for this to form an absolute rule.
It used to be common for "suffix" codes to be used to assist signalmen. For example, adding -2 to a code meant "electric train", letting the signalman at Cliffside know that the train's performance is different and it can't be sent on to unelectrified lines. On busy lines it was also the practice to provide suffices to indicate that a train was going along a particular branch line. These would be passed on as far as the junction box, which would use plain codes to the first box on the branch.
The codes for out-of-gauge loads need more explanation. Suppose that Anytown wants to send a train with an overhanging load towards Downham, and this requires the Up line to be blocked between these two points (because there is nowhere inbetween where another train can pass). Anytown will send the code 1-2-6 to Blackbury. Blackbury will not acknowledge it, but will send it to Cliffside, who in turn will send it to Downham. When Downham is sure that the Up line back to Cliffside is clear and no train is approaching, he acknowledges the code to Cliffside. Cliffside will wait until any train has reached Blackbury, and then acknowledge the code, and so back to Anytown. The signalman there now knows that the Up line is clear all the way. Now Anytown can offer the special train to Blackbury in the normal way, using the code 2-6-3. [If the Down Goods line also needs to be kept clear for this train, Anytown will use the 1-2-6 code on the Down Goods bell. Since the train is proceeding in the same direction, Blackbury can acknowledge it immediately, even if the Down Goods continued to Cliffside (which it doesn't in this case).]
The 2-3-3 code is used for any train that will be proceeding along the Up line (this is the wrong direction, since the Up line is for trains from Cliffside to Blackbury).
Allegedly the code 2-1-2 is also used as an ILC code to protect a trolley that will be going through a tunnel.
The 4-4 code was used in a few places for "crack" trains with special timings. Any delays to these trains had to be reported to higher authority.
The point 440 yards beyond the home signal is called the clearing point, and the line between the signal and the clearing point is called the overlap. If the line is clear to the home signal but not to the clearing point, Cliffside can't accept a train. If the overlap will be clear in a minute or two, Cliffside just ignores the ILC bell until then. However, if it is going to be occupied for some time, it might be preferable to allow the train to proceed anyway. This is done using restricted acceptance, and is only allowed in certain circumstances.
Instead of acknowledging the ILC bell, Cliffside will reply with 3-5-5. Blackbury will acknowledge this code and, when the train is ready to proceed, will warn the driver that the overlap is blocked and extra care should be taken when braking at the home signal.
Only found in Special Instructions. Little information available about when or how this code was used; apparently it has the same meaning as 3-5-5 but was used if Blackbury must not clear any signals. This happened when the two boxes shared a Distant signal which was not slotted; such an arrangement is no longer permitted.
Obsolete code. If permissive working was in use between Blackbury and Cliffside, and there was already a train in the section, Cliffside will use this code in the same way as 3-5-5. The driver must either be cautioned by the signalman that there is a train ahead, or Blackbury will have a special "permissive starter" (usually a small arm which, when cleared, reveals the letter "P") as well as the main starting signal.
Obsolete code. If permissive working was in use between Blackbury and Cliffside, and the line is clear as far as the home signal (but with the overlap blocked), Cliffside will use this code in the same way as 3-5-5. The driver must either be cautioned by the signalman that there is a train ahead.
If Cliffside cannot be sure that the home signal is working properly - and therefore may be showing Clear irregularly - he should telephone Blackbury and inform him. However, if the telephone is unavailable, he sends this code to Blackbury instead of acknowledging the ILC. Blackbury will acknowledge, and will instruct the driver to stop at Cliffside's home signal and contact the signalman.
If Cliffside cannot be sure that the distant signal is working properly - and therefore may be showing Clear irregularly - he should telephone Blackbury and inform him. However, if the telephone is unavailable, he sends this code to Blackbury instead of acknowledging the ILC. Blackbury will acknowledge, and will instruct the driver to ignore the distant signal and assume the home signal is at Danger.
If the signalman at Blackbury can see the distant signal showing Caution, he need not caution the driver.
Normally Cliffside will send the ILC code to Downham when he receives "train entering section" from Blackbury. However, if the section from Blackbury to Cliffside is short, this might not be enough time to set everything up before the train sights Cliffside's distant signal. In this case Blackbury will provide an earlier warning by sending this code (perhaps when he receives "train entering section" from Anytown). Local instructions and knowledge will determine whether and when this code is used.
When Blackbury has obtained permission to send a train to Clifftown, he will send this code as the train enters the section. Clifftown will acknowledge and move the Block Instrument to Train On Line.
When the train arrives at Clifftown, the signalman sends this code back to Blackbury to inform him, and moves the Block Instrument back to Line Blocked. The code must not be sent until the signalman is certain that the whole train has arrived (usually by observing the tail lamp, but sometimes because the guard has waved to say so, or using other techniques).
This code is also used to show that obstructions (such as indicated by the "blocking back" codes) have been removed.
Where the train went from Blackbury to Cliffside on the Up line (having been offered with the 2-3-3 ILC code), this code is used instead of 2-1 to show that the train has arrived complete.
This code is used where a locomotive is to be detached from a train while between Blackbury and Cliffside, or a train is to be divided into two portions. Cliffside sends this code to Blackbury when the locomotive or first portion arrives; it is used instead of 2-1, because the line is still occupied. Blackbury now knows it is safe for the second portion to proceed, if relevant.
If the train stops between Blackbury and Cliffside, and then all or part of it is reversed back to Blackbury, the signalman there sends this code to Cliffside to show that the train or rear portion is back and is clear of the running line. The signalmen must ensure that they agree where all the portions are before another train is allowed on to the line.
If the train has a banking engine assisting it at the rear, Blackbury will send one of these codes to Cliffside after the latter acknowledges the "train on line" code. 2-3-1 is an obsolete code.
If Cliffside has accepted a train using the "restricted acceptance" code, but the overlap is now clear, he sends this code the Blackbury. The signalman there now knows it is not necessary to caution the driver.
Occasionally the box-to-box telephones have to share the same wire as the block bells. If this is the case, the phone obviously cannot be used without warning because it will prevent the bells from working. So if Blackbury wants to talk to Cliffside on the phone, he will send this code.
Blackbury will send this code to Cliffside to test the bells, or to indicate that he is about to test the Block Instrument.
The bells should be tested when a signal box opens. If Cliffside closes and a block switch is used to connect Blackbury straight to Downham, they should immediately test the bells and Block Instruments. The latter should also be tested daily between 10:00 and 11:00. Testing should not take place when a train is signalled between the boxes.
If no train is signalled from Blackbury and Cliffside wants to obstruct the overlap on the Down line - perhaps because of some shunting - he sends this code to Blackbury. He must not allow the obstruction until Blackbury acknowledges the code.
When the overlap is no longer obstructed, Cliffside sends "obstruction removed" (2-1) to Blackbury.
If no train is signalled from Blackbury and Cliffside wants to obstruct any portion of the Down line between Blackbury's section signal and Cliffside's home signal - again, perhaps because of some shunting - he sends this code to Blackbury. He must not allow the obstruction until Blackbury acknowledges the code. Again, Cliffside sends "obstruction removed" when the line is clear again.
This code can be used when the "blocking back inside" code has already been used. In this case the movement on the overlap may continue, but the shunting train must not pass the home signal until Blackbury acknowledges the code.
Where "blocking back outside" has been sent to Blackbury and Blackbury's home signal is less than 800m from Cliffside's home signal, Blackbury may not accept trains on the Down line from Anytown while the obstructing move is in progress. Once the signalman at Cliffside is certain that no further backward movement will happen, he sends this code to Blackbury to indicate that it is safe to accept trains from Anytown.
If Blackbury wants to allow some shunting to take place ahead of his section signal, he sends this code to Cliffside. Cliffside will acknowledge the code to grant permission (and will move the Block Instrument to Line Clear if necessary to unlock Blackbury's section signal).
When Blackbury has sent "shunting into forward section" and the shunting move is complete, with the train once more behind the section signal, he sends this code to Cliffside (who will restore the Block Instrument if necessary).
When a train is signalled on the Down line from Blackbury to Cliffside, but is cancelled for some reason (or is reversed back to Blackbury and will not proceed to Cliffside), the signalman at Blackbury sends this code to Cliffside to show that the train is cancelled.
When a train is signalled on the Up line from Blackbury to Cliffside (using the 2-3-3 ILC code), but is cancelled for some reason (or is reversed back to Blackbury and will not proceed to Cliffside), the signalman at Blackbury sends this code to Cliffside to show that the train is cancelled.
If the signalman at Blackbury realizes he used the wrong ILC code, he sends this code to Cliffside. When it is acknowledged, he then sends the correct ILC code (which should be acknowledged).
Sent when the signals have been restored to danger (unless a train is in the long block) and the block switch - if provided - has been operated.
If Cliffside box is going to close for the night or weekend, but the line is to remain open, the signalman must clear the signals for the main lines. Modern signalling requires, in many instances, that the section signal can only be cleared if the Block Instrument shows Line Clear, so this must be done as part of the closing process.
Assuming this applies to the Up line, Cliffside waits until any train has gone through, and then sends the 5-5-7 code to Blackbury. The signalman at Blackbury moves the Block Instrument to Line Clear and sends a single beat on the bell. Cliffside can now clear all his signals, after which he sends one beat back again. Blackbury now restores the Block Instrument to Line Blocked and sends back the 5-5-7 code to acknowledge that Cliffside is closing. Cliffside will now operate the block switch, connecting Blackbury and Downham boxes together.
If a signal box is closing but a Line Clear release is not needed (either because the signalling does not need it or because no trains will run), this code is sent and acknowledged. If the line is remaining open, the signalman will then operate the block switch and clear his signals.
When a signal box closes, a closing code (either 5-5-7 or 7-5-5) must be sent on each bell. So if Blackbury closes then three codes will be sent, two to Anytown and one to Cliffside.
Suppose that there is an AHB or AOCR level crossing between Blackbury and Cliffside, supervised by Blackbury signal box. If there is a problem, or an Attendant has taken local control of the crossing at the site, drivers approaching the crossing need to be stopped and cautioned. For trains on the Down line the Blackbury signalman can do this himself, but for trains on the Up it needs to be done by Cliffside. So Blackbury sends this code to Cliffside, who now knows to stop trains and caution the driver.
Obsolete code. No information available about when or how this code was used.
Obsolete code. No information available about when or how this code was used.
Obsolete code. No information available about when or how this code was used.
Signalling on single lines is very similar to that on double ones. However, there are a few differences; for example, the codes for working in the wrong direction (such as 2-3-3 and 5-2) do not apply. This section lists those codes that have new meanings for single lines.
Most single lines are controlled using the Token system. The signalboxes at each end have interlocked machines that issue and accept tokens; only one token can be out of the machines at any given time, and a driver may only take a train along the line when he holds a token. Issuing a token requires the co-operation of both signalmen, and is done as part of the "Is Line Clear ?" process.
On a single line, shunting can be carried out without the agreement of the other signal box provided that the train remains within the home signals. If the shunting move will go on to the line, however, permission must be obtained using this code.
It is permitted for both signal boxes to "block back" the same line at the same time. It is also permitted for a shunting move to take place on the line behind a departed train. In this case the 3-3 code is sent only when "Train out of Section" is received from the other signal box.
If a trolley is to work in the section, it will need a token. This code is effectively an "Is Line Clear ?" code for such trolleys.
If shunting or engineering work requires a token to be issued (so as to unlock the starting signal), this code is used to request one. Since the shunting move is protected by the token, the "blocking back" code is not used.
When a token issued for shunting or engineering has been replaced, this code is sent to inform the other end that the line is clear for a train.
On most lines the same number of trains work each way, and so there is no net movement of tokens. However, on some routes or in some circumstances there is an imbalance, and so tokens must be taken from one end to the other to prevent them running out. It is, of course, important to ensure that no tokens are allowed to "escape".If transfers happen regularly, the token machines will be fitted with special transfer magazines. Tokens can be moved from the machine to the magazine through a special connection; the magazine is then taken to the other end, with the tokens locked safely inside, and the tokens moved into the other machine.
However, if magazines are not fitted, the tokens must be carried from one end to the other by the lineman. In this case the signalman with the surplus tokens uses this code to request permission to have the machine unlocked. Once acknowledged, the lineman unlocks the machine, removes some tokens, and relocks it. The number of tokens is written in a special register and countersigned by the signalman. The tokens and register are then taken to the other end and, after checking, are placed in the machine there.
If the signalman at Blackbury requires police assistance and is unable to telephone for it, he sends this code to Cliffside, Anytown, or both.
If train describers are in use the special train description 1-16 has the same meaning.
A train should only pass along the line from Blackbury to Cliffside if the two signalmen have agreed it and Blackbury has cleared his section signal (or instructed the driver to pass it at Danger). If Blackbury becomes aware of a train - or some loose vehicles - running along the Down line in any other circumstances, he sends this code to Cliffside to make him aware of the runaway.
Since the Up line is for movements from Cliffside to Blackbury, normally nothing should proceed along it in the opposite direction (except of course shunting moves after "blocking back" has been agreed, and moves signalled with the 2-3-3 ILC code). If Blackbury becomes aware of a train - or some loose vehicles - running along the Up line towards Cliffside, he sends this code to inform him. The signalman at Cliffside must attempt to divert the runaway on to the right line, and to divert any approaching train away from it.
This code is used for all runaways on single lines.
When a train from Anytown to Cliffside passes Blackbury signal box, the signalman must observe that the tail lamp is lit before sending the "Train out of Section" code to Anytown. If the tail lamp is not correctly in place, or if the signalman was unable to spot it, it is possible that part of the train has been left behind. In this situation the signalman at Blackbury sends code 4-5 to Anytown (instead of 2-1) and code 9 to Cliffside. The former can take steps to check for vehicles left behind, and the latter can stop the train and have it checked.
If Cliffside observes the tail lamp in place and lit, he can send 2-1 back to Blackbury. The signalman there now knows that the train was complete, and so sends the 2-1 to Anytown, effectively cancelling the warning.
If the signalman at Blackbury had seen the tail lamp unlit, but the train is obviously complete, he sends code 9 to Cliffside (to have the train checked) but can still send 2-1 to Anytown.
Whenever a train passes Blackbury, the signalman must examine it to ensure that nothing is wrong and that the tail lamp is present and lit. If he observes a problem other than a missing or unlit tail lamp - such as an insecure load, a door partly open, or a hot axle box - he sends this code to Cliffside, who will stop the train and arrange for it to be checked (the signalman at Blackbury will have telephoned details of the problem). If the code isn't received in time to stop the train, Cliffside will repeat it on to Downham.
If the signalman at Blackbury observes that the train has split, and both sections have passed the section signal (or are likely to), he sends this code to Cliffside. The signalman there can take steps to assist the driver in stopping both parts safely, or can divert the rear part into a sand drag, or other such action.
If the signalman at Blackbury is aware that the rear portion has definitely stopped (or been diverted into a siding) and will not move, he uses the "Stop and Examine Train" code instead.
If the signalman at Cliffside becomes aware that the Down line from Blackbury is obstructed or that there is a possible danger to any oncoming train, he sends this code to Blackbury. The signalman there can then attempt to stop any train. Technically this does not stop trains from being sent from Cliffside to Blackbury on the Up line, but in practice this would not be done until the two signalmen have clarified the situation.
Cliffside also sends this code to Blackbury if a train approaches from that direction (on either line) that is not covered by an previous communication ("Is Line Clear", "Shunting Forward", or "Train Running Away"), or if the train has a banking engine and the appropriate code was not used.
Under normal circumstances the signalmen will know how long it takes a given type of train to travel between the two boxes. If Blackbury has given "Train Entering" to Cliffside, but the train does not arrive until well after that time (or if the signalman at Cliffside becomes aware that the train has broken down), Cliffside will send this code to alert Blackbury to the situation.
If there is an Intermediate Block Section on the Down line between Cliffside and Downham, and a train spends an unusually long time without passing the IBS home signal, Cliffside will send this code to Downham (note that normally this would apply to the Up line, but in this case it applies to the Down).
[All the material in this section is historical and no longer applies.] The Southern Region used different codes for "Is Line Clear" and a few other situations. In particular, there were separate codes for "main line" and "branch line":
|Main line||Branch line|
|3-1||1-3||ILC passenger train|
|2-2||4-4||ILC breakdown train or snowplough going to clear the line or light engine to assist disabled train|
|4-2-2||2-2-4||ILC class 3 or 4|
|2-2-1||1-2-2||ILC class 5|
|3-2||2-3||ILC classes 6 to 9|
|4-1||1-4||ILC class 0|
|3-1-1||1-1-3||ILC engine with one or two brake vans|
|5||5||ILC freight or special train requiring to stop in section|
|2-2-2||2-2-2||ILC trolley requring to go into or pass through tunnel|
|2-3-2||2-3-2||ILC through inspection car or powered trolley|
|1-3-1||1-3-1||ILC inspection car or powered trolley required to stop in section|
|2||4||Train entering section|
|1-4-1||1-4-1||Engine assisting in rear|
|1-5-1||1-5-1||Engine and one or two brake vans assisting in rear|
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