The Second Postulate (3)
Here Einstein's reasoning is wrong, because he used the first postulate in the opposite direction.
From the previous section, we know that relative motion is a must for the first postulate.
Just as in the example of the apple falling, the first postulate ensures that accelerations from all reference frames are the same, while the velocities are different.
Here, the observed speed c-v on the train is exactly what the first postulate demanded.
Observing the same velocity from all reference frames will destroy the first postulate completely.
So, here is our conclusion on the second postulate:
"The speed of light is the same in all reference frames." is the result of incorrect reasoning.
* Many people take this incorrect result as the second postulate itself.