Varies by place but the interlocking will let the route be set regardless of signalling direction, it will be the signal interlocking itself that checks the occupation (or lack thereof) of the section ahead, then a valid route is set and then shows a proceed aspect.
Theoretically if the passenger train was 200 meters back and behind a signal, the signaller could route the trains into each other, but the signal wouldn't clear (if everything is working correctly)
Essentially what im saying is the signalling doesn't know which train will move next and so allows a route to be set as long as all other prerequisites for the route are met.
If an individual points switch is used, the interlocking doesn't care what is around the points, as long as nothing is ON the points. And will let you move them until you get bored doing so.
The lay of the points as they are here (assuming both ends were in reverse as they are designed to be) is a perfectly valid position for the points to be in and that's all that matters for them to be moved.
The signal however, when you try to clear it, will then see it either isnt a valid route, or that there is already a train there. And not clear.
That would definitely be the case if they were controlled by a mechanical signalbox, but Clay Cross North junction is controlled by the East Mids Control Centre so works on the principle of setting routes, rather than operating individual points / signals.
Note, for ease of wording, where I say track circuit, it could be an axle counter section instead.
For route relay interlocking (PSB / Panels) and later computer based interlocking systems the following general principles apply:
Points
Power operated points must be electrically locked if the track circuit that runs through that set of points is occupied. For multi-ended points (e.g. either end of a crossover) all of them will be locked when any track circuit over any end is occupied.
If no routes are set over the points, and all approach locking is free (meaning no approaching train or the possibility of an approaching train because a signal has been cleared and then put back to red) and the relevant track circuits (as described above) are clear, then the signaller can operate the points via the individual point switch (IPS) or the points can be automatically be called via the route calling relay (or equivalent) which operates when the signaller routes a signal.
If point to point locking or flank protection is being used in the interlocking, then that may restrict which points can be moved.
When the above conditions are met, the signaller can put the points in any position they want. However, multi-ended points will all be called to move at the same time as they are all controlled by the same IPS (e.g. both ends of a crossover).
Routes
The interlocking checks if the signaller is trying to set a valid route. If what the signaller is trying is not a valid route, nothing will happen (although with some systems, e.g. computer based interlocking, the entry button light will light up when pressed, but then go out when the interlocking rejects the invalid route).
If a route has already been set, and the signaller tries to set an opposing route (i.e. route two trains in a converging move on the same line) into the same signal section, the interlocking will reject this. This is called opposing locking. Note that it does not stop the signaller (or ARS) from setting a route towards another train if there is a complete signal section that is free of route locking.
If a route has already been set, and the signaller tries to set a converging route, the interlocking will reject this.
When setting a route, the position of all the points is checked by the interlocking. If any are not in the correct position, the interlocking calls the points to move. It then checks that they have all moved to the correct position. If any have not been detected as being in the correct position the signal will not clear (signallers call this detection, S&T call this correspondence). By correspondence, S&T mean: the points have been called to the correct position, and they are detected (proved) to be in the position they were required to be called to including all ends of multi-ended points.
Note that in most systems, the route calling relay (or equivalent) ignores the status of track circuits. hence a route can be called even if there is a train in the section. However, if the track circuit directly locks any points or the track circuit is maintaining the route locking any and all points held electrically locked by this will not be called to move.
Signal Interlocking
The signal will only clear if all the relevant track circuits are clear and all the relevant points are detected in the correct position. As soon as these conditions are no longer true, the signal will go back to red.
Point Failures
If a set of power operated points cannot be operated on power or there is any other reason why power operation is not possible, (including track circuit failure, point detection failure, the facing point lock failing to operate), then the points can be operated manually.
However, if detection cannot be obtained, then the signal will not be able to be cleared. In which case, the signaller must advise the driver of the circumstances and give them permission to pass the signal at red. Of course, the MOM (or other route setting agent) has to ensure that ALL the points are in the correct position. There are box instruction that should list the required point positions for all valid signal routes.
And yes, it is also the job of the train driver to check that the points are set correctly when this happens. So the train should proceed at a suitable slow speed so that the driver can do this.
Please note that I have not included all the details. I think I got all the important ones. Other forum members please shout out if I've missed anything.
Edited to correct the word "working" with "wording".
