Signal 451 green with train immediately in advance

In the case of the carriage washer there can be two trains in the non-track-circuited track. How can be be certain that with last-wheel control, that the signal will not be viewed by the following train?
I'm fairly certain that I've gotten messages from the second train the washer about an adverse change of aspect, but now that I'm looking for them of course it isn't happening!

The guard is less of an issue, because they can expect signals to be put back before the back of the train reaches them.

Preserved railways tend to expect their Guards to observe every signal, irrespective of where it is.

When I was a Signalman on the Severn Valley, we used to keep the signals off until not only the entire train had passed the signal, but until the entire train had passed over any points to which the signal applied. If we replaced the signal before the train had completely passed it, the Guard would be expected to apply the emergency brake.

Mechanical areas, certainly those without track circuits, rely on the position of the signals as the only "check" on the movement of trailing points (facing points are also protected by depression bars), so returning a signal over a trailing point could allow you to move it under a train, which is not desirable.

With the introduction of track circuits, this latter function was not required, since the points would be locked by the track circuit being occupied, and some form of route locking was generally applied as well, even if this was not visible to the Signalmen. For instance, in Cornwall where I work now, the installations dating back to the 1960s have electrical locking on the points, so even by replacing the signals to danger the approach locking and/or route locking would hold the point levers where they are preventing any movement in front of or under trains.

Where MAS is in use, the actual aspect of the signal has no real bearing on the locking of points as the approach locking and route locking do the holding of the points.

I am not up-to-date with rules that train crew have to follow, but if memory serves me correct the only thing the guards and dispatch staff have to satisfy themselves of is that the signal at the end of the platform is showing a proceed aspect when they give right away, and after that it is the sole responsibility of the driver to obey any signals. There are many type of unit (such as Voyagers) where there are no facilities for Guards to view signals once the doors are closed anyway.

How the signal is replaced is not really all that relevant as there would be little difficulty in designing every signal to be last wheel replaced if the rules so demanded, and in many early MAS installations platform starting signals often were designated last-wheel replacement.

It is possible that the treatment of signals being replaced before the train having totally passed them as an emergency signal may have been a symptom of the need to maintain the route locking rather than the initial reason for the signals being kept off. I know that it is common practice on some parts of the national network to replace semaphores before the back of the train has passed them completely, although in other places it does not happen.