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Added description of instruments and tokens
With the staff and ticket system, there was little to stop the signalman issuing tickets improperly. Token systems were developed as one way around this problem, using electrically-connected machines called 'token instruments' to hold a set of objects like the earlier train staff, called 'tokens'. The machines were designed to be linked in pairs, with each pair of machines sharing a set of tokens, and designed so that only one token in the set can be removed from the instruments at any time. The first token instruments used metal batons ('electric train staffs') or discs ('tablets'), and tokens always had to be removed from one machine and returned to the other. The SVR uses a later type of instrument called a 'key token' instrument, so named because each token resembles a large key.
== The Token Instruments ==
The largest part of each token instrument is a 'magazine' consisting of four vertical channels, which stores each machine's stock of tokens. Because each token has a large head, wider than each channel, they can be moved around inside the magazine but not removed. Above the magazine is a lock mechanism called a 'commutator'. This is the only route by which tokens may be taken from or placed into the instrument, by turning the token like a key in a lock.
Alongside the commutator is a bell plunger used to signal to the other signalbox, a telegraph needle which moves when the plunger in either instrument is pressed, and an indicator which reminds the signalman whether a token has been released or not. Some token instruments have automatic electrical indicators, but all of the instruments on the SVR have manual ones.
Tokens can be placed into an instrument at any time, if they fit, but a token can only be removed if all the tokens are in the two instruments, and the signalman at the other signalbox is holding down the bell plunger on his instrument.
Each instrument is numbered, and each token, like a key, only fits into the correct instrument. The tokens are also stamped with the names of the signalboxes at either end of the section they apply to, and are colour-coded, with all the tokens in a set having their handles painted a distinctive colour.
== See Also ==
