A ‘Token’ is a physical object which the Driver of a train is required to have before entering a particular section of single track. The token is clearly marked with the name of the section it belongs to. A token system is used for single track lines because of the greater risk of a serious collision in the event of irregular working by signalmen or train-crews, as compared to double track lines.
The operation of a bidirectional single track line has a few problems, the most serious of which is the possibility of two trains traversing the line travelling towards each other, each driver unaware that the other is using the line. The simplest method of controlling such a line is to have only one train operational, on the basis that a single train cannot collide with itself, and in the absence of another train, there is nothing else for it to collide with. Such a system was known as ‘One Engine in Steam’. Such schemes were used, and indeed still are used on some branches of rail networks, and on various heritage railways. The main problem with such a scheme is that it is best suited to a completely isolated branch of single track line. Where the section has to be integrated into a larger railway system, it becomes exceptionally limiting in the level of operations that it allows, and the opportunity for a mistake to be made, and an ensuing accident to occur, is high.
Instead, reliance is placed not on employing only one engine but on having a single physical object available for the single track section, and ruling that only if an engine driver is in physical possession of that object may they enter the single line section. That object is known as a token and is marked to indicate to which single track section it belongs.
Tokens have existed in a variety of physical forms: