Well yes, but now you have completely lost me. I no longer have any idea where you are going with all this.
I have been maintaining from the beginning that what counts in all these theories is what is observed. Certainly, the classic form of this "train" thought experiment focuses on what is observed by S and P when the reflected light reaches them, rather than on anything they might imagine to be happening before it does so. (Light reflected from the front of the train to S will takes longer to arrive than it does to P, because for S the front of the carriage has moved away a bit before the light hits it, so the path it take is longer than the path back to P, who is moving with the train, etc, etc. This is all standard.)
But carry on and let me see if I can follow where you go next.
So, thank you for listening.
I will try to summarize. We know from Quantum Mechanics and various experients (e.g. Hiatachi) that by taking a measurement we "collapse" the wave front. Actually i don't like that word, but we can move past that.
So we are looking at the train experiment (We could look at other experiments but this works well enough) from a perspective of who sees (measures) what and when.
I believe that a moving wavefront or photon cannot be observed and that the only observable event is the image created when that light illuminates something.
Now we have just agreed, I believe, that when the light strikes the front of the train both observers receive that image and that it is identical for them.
What happens next is your question. Well the next observable image in this scenario is that the light arrives at the Passenger, travelling a distance L/2 at speed = c. This also creates an image and that image travels from that position to the Stationmaster (S). It travels at the speed of light (it is light) and arrives at the Stationmaster.
Now the Stationmaster sees that image before he is illuminated by the same flash. I think that is what you just described.
The thing here is that he sees it at some distance from him. Since we were 8 years old, or so, we have been counting 1000 & 1, 1000 & 2, etc. In order to determine how far away a lightning strike from us, so our 8 year old Stationmaster can do the same thing and say - yes, that happened 3 seconds ago, but it happened x distance from me so it actually happened at the same time. In fact motion is not required. Given that all we have seen is two illuminations we can simply conclude that if someone were standing on the platform at the position where we see the Passenger illuminated then they would identically see the flash first, and we can calculate that, and that the difference would be a function of the distance travelled in time t at velocity v, and that the apparent difference can be determined by counting 1000 & 1, etc. just as we did as kids.
L is the same, c is the same and t is the same.
The fact is SR uses light as our measuring tool and our tool has a time dependency so taking a measurement of the position or behavior of a moving object is dependent on whether it moves during the time we take the measurement.
Is there somehing Here you disagree with?
If not I will continue.