Token (railway signalling)
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On railways, a token (also staff or tablet, depending on its shape) is a physical object which a locomotive driver is required to have before entering onto a particular section of single track.
The simplest token system was developed in Britain in the 19th century. In this system only one token exists, so the driver holding it has absolute assurance that no other train is authorized to enter the section.
However, this method does not provide sufficient flexibility when a railway line consists of several single-track sections separated by passing places: consecutive trains could not be 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 see the staff or 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, but this was either a section of staff designed to be removed for the purpose, or a written ticket. To ensure that the ticket is not issued incorrectly the tickets are placed in a locked box, they key to which is permanently fastened to the token. In addition the lock prevents the token being removed until the ticket box is closed.
This 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 were developed. Each single-track section would now be provided with a pair of these machines, one at each end of the section, and a supply of identical tokens stored in them. 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.
The final development of this system was to interlock 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.
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 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.
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 protected section have the levers padlocked. The key required to unlock the point and move it is also permanently fixed to the token. Only when the point is physically back in its 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 point levers one per token required.
Without these safety checks it would be possible for a train to arrive at the end of the token signalled 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.