Token (railway signalling)
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On railways, a token is a physical object which a locomotive driver is required to have or see before entering onto a particular section of single track. The token is clearly endorsed with the name of the section it belongs to.
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[edit] Forms of Token
Tokens have existed in a variety of physical forms:
- Staff.
- Tablet.
- Ball.
- Key.
[edit] Token Only
The token system was developed in Britain in the 19th century. In its simplest form, only one token exists for the single line section, so the driver holding it has absolute assurance that no other train is authorised to enter the section. When the train reaches the end of the section, the token is left at that location, ready to be used by the next train going the other way. In UK terminology, this method of working was originally referred to as One Engine in Steam (OES), and later One Train Working (OTW). In a system such as this where only one token exists per section, the token is usually referred to as the train staff, regardless of its physical form.
[edit] Token plus tickets
Using only a token does not provide sufficient flexibility when there are to be consecutive trains worked in the same direction. The simple token system was therefore extended. If one train was to be followed by another in the same direction, the driver of the first train was required to be shown the token, but not take possession of it. This provided assurance that if there was another train in the section, it was not travelling towards the first train. To avoid mistakes, the driver did have to carry a physical object with him, in the form of a written ticket. To ensure that the ticket is not issued incorrectly, a book of numbered tickets is placed in a locked box, the key to which is permanently fastened to the token, or is the token. In addition, the lock prevents the token being removed until the ticket box is closed, and it cannot be closed unless the book of tickets is in the box. Once a ticket is issued, its number is recorded in a Train Register book, and the token is locked in a secure place. This system is known as train staff and ticket. In a variation on this principle, called divisible train staff, a section of the token referred to as the ticket portion was designed to be removed and handed to the driver instead of a paper ticket.
The train staff and ticket system alone will prevent a head-on collision, but some means is required to avoid collision between following trains that are to run through the section in the same direction. Originally, time interval working was employed, this being superseded by block working when that became a mandatory requirement. With the latter system, when the train reaches the end of the section, a message is sent back by telegraph or telephone that the train has arrived. The message includes the number of the ticket, so the signalman can check that it is the correct train, and record the details in the Train Register book. Once this message is received, the signalman can remove the token from its locked location and make it available for the next train, or for issuing the next ticket.
[edit] Electric Token
The train staff and ticket system was still too inflexible for busy lines, as it did not allow for the situation where the train intended to carry the actual token was cancelled or running very late. To provide for this, electric token-dispensing machines, or instruments, were developed. Each single-track section would now be provided with a pair of these machines, one for each end of the section, and a supply of identical tokens stored in them. These machines would be connected together electrically and using simple electromechanical logic, either machine would dispense one token when requested by the signalman, but only if the two machines contained the entire supply of tokens between them. Once a token was out, it had to be put back in one or the other machine before another could be taken. By this means, it was guaranteed that at any one time, only one token was available to be issued to a driver. Tokens belonging to adjacent sections have different configurations to prevent them being inserted into the wrong instrument.
[edit] Collection of the token
If a train was not otherwise required to stop, a Token could be handed from the signalman to the driver of the train, or vice versa, without the train stopping, although it had to be travelling slowly. If the train is to proceed on a Ticket, however, the driver usually needs to stop so that the driver can both collect the Ticket (difficult to do on the move, being a small piece of paper) and inspect the Token to check that it is the Token for the section he is about to enter.
In order to facilitate the collection of the token, they would often be attached to a hoop that can be more readily retrieved by it being caught on the outstretched arm of the driver or signalman.
On some railways which used an electric token system on main lines, locomotives were fitted with a hook shaped roughly like a ram's horn. Approaching the start of a Token section, the driver would lower the hook, which would catch the hoop holding the token, from a stand at the side of the track where the signalman had placed the token. By this means, drivers could collect the token from the signalman without slowing down.
[edit] Intermediate Block Posts
The stations at each end of a Token section are known as Token Stations. There might be times that the token section is too long for the desired interval between trains, so an intermediate station is opened as an Intermediate Block Post thus dividing the section into two. A train would be issued a ticket for the Token section A to C, but on arrival at B the train arrival message would be sent back to A, allowing another train to follow. When the first train arrives at C, the train arrival message is sent back to B. When the second train arrives at B, it is not allowed to proceed unless the train arrival message has been received from C.
[edit] Intermediate Token Stations
An Intermediate Block Post is useful for a second train following the previous trains at a short interval, but is no use for running more trains in opposing directions. The solution here is to open an intermediate station which has a crossing loop as an Intermediate Token Station. In this case, tokens would be provided for the sections A to B and B to C. The facility may be provided that will allow the Token Station at B to be 'switched out' or 'closed' when not required, and 'long section' tokens for the section A to C to be used instead. However, in order to preserve the integrity of only one token being provided for each section, when B is opened as a Token Station, the token for the section A to C is withdrawn from use and replaced by tokens for the sections A to B and B to C. When B is closed as a Token Station, the reverse occurs. (Working examples of this scheme may be found between Bridgnorth and Highley (via Hampton Loade) and between Highley and Bewdley North (via Arley) on the Severn Valley Railway in Shropshire, England.
Sometimes a special Token box is provided that holds the three tokens, in which an internal mechanism will release either the A to C token or the other two tokens, but not all three at once.
[edit] Signals
A development of the token system was to 'interlink' the tokens with the signals, so that a train could not be signalled onto a single-track section unless a token for that section had been dispensed from the machine at the same end of the section. This would usually take the form of a "one-pull" interlocking. Having removed the token from the token instrument to pass to the driver, the signalman would only be able to pull the section starter signal once. I.e. when returned to danger it would be electrically locked until another token was withdrawn from the instrument at that end of the single line section.
[edit] Shunting in intermediate sidings
The token may be used to unlock points or a ground frame (see Token interlocking, below) at an intermediate siding. However, in the absence of some means of restoring the electric token instruments to their normal state, the single line section will not be able to carry other traffic whilst shunting takes place at the siding.
Rather than have the token unavailable for other trains during this shunting, an Intermediate Electric Token Instrument can be provided near the points of the siding. These instruments are similar to the machines at each end of the section, but are usually only capable of holding one token. A train arriving at the siding would use the token to unlock the points then move into the siding.
Once the whole train is clear of the main line, the points would then be reset for the main line and the token inserted in the intermediate token instrument. In UK practice, this is called "shutting in". Another token may then be withdrawn from one of the main instruments at each end of the section, to allow trains to continue running whilst the siding is being shunted. When the shunting is complete, the token is withdrawn from the intermediate instrument (if no other token is out of an instrument already) and the train can use it to return to the main line.
[edit] Pilot working
A variation of the token system is pilot working, where the place of the token is taken by a person who is designated the pilotman. This person rides on the locomotive in lieu of the driver taking the token. If more than one train needs to proceed in a given direction before a train returns, the pilotman can instruct the driver to proceed through the section, and then ride on the last train in that direction.
Pilot working is used as a temporary replacement should the token instruments become defective, for a missing token or to work trains during single-line working.
[edit] Token interlocking
Railway regulation in most countries require that points (US: switches) are interlocked with the signalling system. A token must provide the same guarantees as other systems. To achieve this any points on a token-controlled section have the levers locked. The key required to unlock the points and move them is the token itself, or a key permanently fixed to the token. Only when the points are physically back in their normal safe position can the key (and token) be removed from the lock.
Where two tokens are required (such as entering another token protected branch or crossing another line) there are two or more locks on the points lever: one per token required.
Without these safety checks it would be possible for a train to arrive at the end of the token-controlled section of line having left the points in a state that would direct the next train into a siding or derail it. These checks and locks ensure that possession of the token also means the points are in their expected positions and that token possession really does indicate the section is safe to traverse.
[edit] Current use
Until the late 20th century the token system was standard on British single-track lines, and it still remains on a number of lines there as well as elsewhere. However, its use is declining due to the development of electronic signalling systems. The electronic systems themselves often use virtual tokens and one of the most common systems replacing the traditional token is Radio Electronic Token Block, a wireless implementation of tokens where the token is transmitted by radio to the train.
The token method of working is sometimes still used for temporary situations such as through engineering works or where the signalling system has yet to be provided. In December 1981, a new underground line was opened in Melbourne, Australia which provided a circular service around the city's central business district. Although normal services were not to begin for some time, for two months special services operated on Sundays to allow people to try the new line, and for this period the line operated with a Train Staff, although there was no provision for tickets to be used.
A very unusual token working was instituted on the line between Pakenham and Traralgon in Victoria, Australia for several month in 2006 whilst the line was being upgraded. During this period, the line was closed during the day, but opened each night to allow a few goods trains through. The line is double track between Pakenham and Moe (except for a short single track section). Three Staffs were provided for the 100km length, one each for the sections Pakenham—Warragul, Warragul—Moe, and Moe—Traralgon. The first two of these are partly or totally double-track sections, but the staffs in this case were applicable to both tracks, the effect being that only one of the tracks could be used at a time.
[edit] Unattended operation
Tokens can be arranged for unattended operation, whereby train crew operate the token instruments at intermediate crossing stations. This is widely found in Australia, where traffic density on many lines is low. A UK example is the North Devon Line, where the system was brought into use on December 1 1987. A corollary is the automatic crossing loop.
