   in railway signalling, 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. a token system is used for single lines because of the very much greater risk of serious collision in the event of irregular working by signalmen or traincrews, than on double lines. == principle == thumb|right|examples of single line tokens: tablet on left, key token on right the operation of a bidirectional single track line has obvious problems, the most serious of which is the possibility of two trains traversing the line travelling towards each other, both drivers unaware that the other is using the line. the simplest method of controlling such a line is to have only one engine operational, on the basis that a single train cannot collide with itself, and in the absence of another engine, there is nothing else for it to collide with. such a system is known as "one engine in steam". such schemes were used, and indeed still are used on some branches of rail networks, and on 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, rather than rely on a single engine, reliance is placed 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, is he permitted to enter the single line section. that object is known as a token and is marked to indicate to which single track section it belongs. == forms of token == tokens have existed in a variety of physical forms: * staff * tablet * balldescription of neale's ball token instrument * key == token systems == === token only === the token system was developed in britain in the 19th century, to facilitate safe working of single-line railways. if a branch line is a dead end with a simple shuttle train service, then a single token is sufficient. the driver of any train entering the branch line
Railway_signalling_block_systems