Signalling block systems enable the safe and efficient operation of railways, so as to avoid collisions between trains. Block systems are used to control trains between stations and yards, and not normally within them. Any block system is defined by its associated physical equipment and by the application of a relevant set of rules. Some systems involve the use of signals, while others do not. Some systems are specifically designed for single track railways, where a danger exists of both head-on and rear-end collision, as opposed to double track, where the main danger is rear-end collision.