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
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:
Whenever conditions indicate that a route has cleared up to
the boundary, East issues a remote cancellation request to West.
When West receives a request to cancel an inward route it
does so unconditionally, and acknowledges the request with a reply
telegram to East.
On receipt of such an acknowledgement, East should cease
to issue cancellation requests, the route having been cancelled in
Matthew Morley, Edinburgh. Date: 29 November, 1998