Einstein introduced the train-hit-by-lightening thought experiment to illustrate "the relativity of simultaneity". For the three reasons below, I think he was wrong when he held that the onboard observer sees the forward flash before seeing the rear flash. I realize I'm challenging the Great Man himself as well as every author who's described this scenario. I'm looking to resolve my dilemma, not to pick arguments with authorities.
No experiment within an inertial reference frame can distinguish between "movement" and "rest".
If Einstein's train were at rest relative to the tracks the onboard observer would see the two lightening strikes as simultaneous.
According to the conventional (Einsteinian) explanation, with the train in motion relative to the tracks, the onboard observer sees a temporal gap between strikes, the forward strike appearing to have taken place before the rear strike.
Therefore, by noting whether the strikes appear simultaneous or sequential, the onboard observer can discern the difference between the absolute rest and the absolute motion of the train.
Point 3 violates SR in that no experiment performed in an inertial frame of reference can determine the frame's absolute rest or motion.
Further, the length of the gap could be used to calculate the train's absolute speed. In his 1948 book, The Universe and Dr. Einstein — with an introduction by Einstein himself — author Lincoln Barnett says:
"Imagine temporarily that the train is moving at the impossible rate of 186,242 miles per second, the velocity of light. In that event, flash B [the rear flash] will never be reflected in the mirrors at all because it will never be able to overtake the train."
The author's obvious implication is that the gap between the two flashes is an indication of the train's speed.
There is no preferred reference frame.
One of the tenets of SR is that the laws of physics are the same in all inertial reference frames. In Einstein's thought experiment both the train and the embankment are inertial reference frames. Neither is accelerating. This means that the train and the embankment are interchangeable: We can assign "movement" to either one and "rest" to the other without altering the conditions of the scenario in any way as long as the relative motion between the two conforms to the original specification.
So, let's assign "rest" to the train and "movement" to the embankment. Now, surely the onboard observer will see the two lightening flashes as simultaneous. (We're not concerned here with the trackside observer.) If the onboard observer sees the flashes as simultaneous under one scenario he cannot see them differently in the other, equivalent scenario. Our arbitrary choice of which inertial frame to consider "moving" cannot alter the physical result of the experiment.
Einstein's train experiment incompatible with Greene's.
In his book, The Elegant Universe, author Brian Greene presents an alternative thought experiment to Einstein's. In Greene's version two kings, sitting at opposite ends of a long table on a moving train, attempt to simultaneously sign copies of a peace accord. Neither wants to go first. So they agree to begin signing when an electric lamp at the table's midpoint is illuminated.
The onlookers on the train all agree that the two kings signed simultaneously. But those on the embankment complain that the forward-facing king signed first because, while the light from the lamp propagated toward the kings, the train's forward motion brought the forward-facing king closer to the source of the light, while the backward-facing king receded from it. Thus, the light had less distance to cover to reach the forward-facing king than to reach the backward-facing king.
If, in the Einstein experiment the onboard observer saw the forward lightening flash first because the train carried him closer to its source, why, in the Greene experiment didn't the onboard observers see the forward-facing king sign first for the same reason? They didn't; they saw the two kings sign simultaneously.