An interlocking is the safety-critical low-level control system in railways, tasked with ensuring that all actions involving signals and movable elements are performed in a safe manner.

The main operating mode of the interlocking is to take requests for setting a route, and then moving track elements and giving signals if this route is safe to set. Routes are the atomic unit of allocation from the dispatcher's point of view. Note that the atomic unit of de-allocation is typically smaller than the route, a feature of interlockings which is called partial release.

The routes file contains set of whitespace-separated route statements, each declared by the following syntax:

route <name> {
  entry <signal-name>
  exit <signal-name>
  entrysection <section-name>
  length <number>
  sections [<section-name>, ...]
  switches [<switch-name> <switch-position>, ...]
  contains [<node-name>, ...]
  <release>*
}

The lines in the above route statement contain the following:

• A route has a name, which is the identifier used from the dispatch plan to request this route for activation.

• The entry is an identifier for a signal which is used to signal the movement authority given by this route when it is activated.

• The exit is an identifier for a signal which is not directly used for simulation, but which is need for planning for knowing which routes can extend this route.

• The entry section is an identifier for a detection section. When this detection section becomes occupied, the route is considered to be in use, and the signal stops signalling movement authority to avoid other trains from using the same route before it has been activated again.

• The length is a number indicating the driving length of the route path. This length is used by the planner, and also by the simulator in the communication of movement authority from the control system to the train, in case of a signalling system which transfers movement authority lengths (either on-line, through train protection systems or radio communication, or off-line, by having the train driver familiarize with lengths though a railway line's handbook). This length is not adjusted for stopping margins.

• The section list is a comma-separated list of section which must be exclusively allocated before the route can be activated. Sections should be listed in the order of occupancy by a train running the route path.

• The switch list is a comma-separated list of switches and corresponding positions (e.g. sw1 left).

• The contains list is a comma-separated list of node which the route contains. It is used for planning, to fulfil requirements of visiting specific nodes by activating routes.

• Finally, a list of release conditions determine how the resources required by the route are de-allocated. The syntax is as follows

  release {
    length <number>
    trigger <section-name>
    resources [<resource-name>, ...]
  }
  

  The lines in the above release statement contain the following:

  • The length is a number indicating the length of the partial route path.
  • The trigger is an identifier for a detection section. After this trigger has turned occupied and then subsequently vacant again, the resources are released.
  • The release list is a comma-separated list of resources, i.e. sections and switches, which cease to be allocated after the trigger event has occurred.

  Note that if no releases are specified, a default release condition is added which frees the whole length and all resources, using the last section in the path as the trigger.

Two special cases of route syntax exist for the entry and exit from the model.

modelentry <name> from <boundary-name> {
  exit <signal-name>
  (length, section, switches, contains as in route)
}

modelexit <name> to <boundary-name> {
  entry <signal-name>
  entrysection <detector-name>
  (length, section, switches, contains as in route)
}

Many railway administrations require a given length of track after the exit signal to be vacant or exclusively allocated for a route to be activated. This track section is called overlap (british) or safety zone (international). Overlaps require special logic for allowing the extension of a route

Overlaps are currently not supported.

Flank protection can be implemented by adding requirements of exclusive allocation of resources such as detection sections and switches.