Einstein’s famous train thought experiment asks us to imagine two lightning bolts
striking the ends of a moving train simultaneously — as observed from the embankment.
A passenger at the centre of the train, however, sees the bolt at the front first.
Einstein concluded from this that simultaneity is relative.
But here is what troubled me for years: the thought experiment assumes light travels
at c relative to the embankment observer. It then derives the consequence for the
train observer. The conclusion — that simultaneity is frame-dependent — follows
logically from that assumption.
What the experiment does not do is prove the postulate itself. It demonstrates
consistency, not truth. Einstein was careful about this distinction in 1905. Many
who followed him were not.
This is precisely the gap that Kinematics of Moving Bodies attempts to address.
Not by abandoning the postulates — but by following them more rigorously than
has been done before, and seeing where that rigour leads.
The answer, it turns out, is somewhere quite different from where Einstein arrived.