The signalman or control system calls a route which the interlocking then prepares. The route is <i>locked</i> either after a given <i>delay </i>but more generally after an <i>aproach section </i>turns occupied. This indicates an approaching train that <i>triggers </i>the route locking. The trigger is typically <i>assigned </i>to a set of TVP sections.<br/><br/>The route approach lock can apply to <br/><ul>
<li>an entry signal, thus locking the proceed aspect of the entry signal</li><li>a (set of) route(s) so that the triggering mechanisms depends on the kind of route, e.g. entry speed</li><li>a route body in case the aspect or other qualities of the route are irrelevant</li></ul>
<br/>A route can be cancelled until the approach lock has been triggered. After that, the route can only be released given a specific procedure.<br/>When the train is close to the route entry signal, some systems can prevent the signalman from cancelling the route.<br/>There can be points and signals in the approach zone. This model doesn't cover how the state of signals and points influences approach locking. <br/><br/><br/>