Isn't the use of sound fundamentally different from electromagnetic signals? For enclosed trains, any observer will agree that the speed of sound is the same relative to and inside each train. With electromagnetic signals each observer finds the speed of light the same relative to themselves. I have a feeling that this is another of those situations where we are all actually in agreement but talking past each other because we have slightly different examples in mind.
John Denker Wrote:
You can get along just fine without the star, if you are able to
send a signal up and down your train in a way that is independent
of the state of motion of the train. A sound signal suffices,
if your sound is very very accurate, and the medium is comoving
with your train, and you invoke Galilean relativity. In practice
people often use electrical pulses.