1.4.2. Releasing Sub-routes over Boundaries

Once a train has passed the entrance signal and progressed along the route (or if the route is subsequently cancelled by the signal operator) the sub-routes along it should be released in the usual manner---i.e., by rules in the continuously executed sub-route release data (as in Section 1.3.3). At least, the sub-routes can be freed in this way up to the boundary: T8ab is a control variable in East of course, while T7ab is in West, so the usual rule for freeing the `inward' sub-route does not apply.

In order for the whole of the route to become free East must send a request to West for it to release its part of the route once the correct circumstances obtain. If the sub-route release mechanism is to be transparent (to the operators) the necessary cancellation request should be issued automatically. To achieve this in SSI the correct circumstances are recognised by rules in the sub-route release data. If West receives a cancellation request it can release the inward portion of the cross-boundary route unconditionally. Furthermore, West should acknowledge the cancellation request so that East will be aware that the route has indeed been released. The usual sub-route release mechanism in West will ensure that the remainder of the route is released as the train proceeds to the next signal. To summarise:

: 4 Whenever conditions indicate that a route has cleared up to the boundary, East issues a remote cancellation request to West.
: 5 When West receives a request to cancel an inward route it does so unconditionally, and acknowledges the request with a reply telegram to East.
: 6 On receipt of such an acknowledgement, East should cease to issue cancellation requests, the route having been cancelled in both Interlockings.

Matthew Morley, Edinburgh. Date: 29 November, 1998