Approach Locking

The locking of any Route from a Signal, when the driver has seen or may
have seen a Proceed Aspect at the Signal or has seen an Aspect at a
previous Signal that would indicate to the driver that the former Signal is
displaying a Proceed Aspect. If the Signal is replaced to danger, the
approach locking prevents the immediate release of the Route because it is
possible that an approaching train may be unable to stop.