A year ago I posted about observing clocks on
a very fast locomotive moving between two
fixed stations and how the need to see the same
amount of blue or redshift from each station
would lead to the requirement the moving clock
would record less time than the station clocks.
Now I have tried it with sound. If the moving
train emits a tone going at half the speed of
sound between two stations I find the distant
station will hear a period of sound that was
emitted while the train was stationary then
will hear it at twice the frequency while
coming. But the train driver would hear the
same tone from the fixed distant station at
one and a half times the frequency while he
moves. Its surprising, the principle of
relativity is so engrained and I never learned
this about sound. The sound of crossing bells
on a train heard in so many old films must
have a higher pitch if put on the train and
heard at the crossing! Ever been tried?

This isn't the principle of relativity, this is Doppler shift.

That's right, and the key difference is, sound requires a medium so the central premise of relativity is not in place (which is that only relative speeds of the source and observer is relevant). With sound, there is the speed of the train with respect to air, and the speed of the observer with respect to air, so there are two speeds in the problem with different results.