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,Reworded bit about trains entering without tokens
Its most easily understood form is that where a token instrument is installed in the signal box at each end of the single line section thus forming an electrically connected pair. Each instrument has an electro-mechanical arrangement such that only one token can be removed from one of a pair of instruments with the cooperation of the signalman at the other end of the single line section. No other token can be removed until the originally removed token is replaced in one of the two instruments.
A token gives the driver of a train authority to occupy the single line section. Only in very exceptional circumstances may another train occupy or a enter the section without being in possession of a token, or two trains be present in the same section.
The SVR uses Tyer's Key Token instruments. http://tillyweb.biz/gallery/aa/arleyekts.jpg
