Keith,
Could it be something to do with the Doppler Effect?
http://en.wikipedia.org/wiki/Doppler_effect
"the change in frequency <http://en.wikipedia.org/wiki/Frequency> of a
wave<http://en.wikipedia.org/wiki/Wave>for an observer moving relative
to the source of the wave"
best
luke
2009/11/19 Keith Russell <[log in to unmask]>
> Dear Listers
>
> Perhaps there is someone out there who can relieve me of my morning puzzle.
> It is stopping my normal activities.
>
> While walking to work, I pass by a train line that carries the coal trains
> that deliver the largest amount of export coal in the world.
>
> This morning, an empty coal train came to a stop nearby. It then started to
> move again.
>
> When it stopped, I noticed the usual ripple of sound that accompanies the
> concatenation of carriages as they shunt into each other.
>
> When it started up again, the auditory ripples were much more pronounced
> and they repeated in both directions for some time.
>
> What I noticed was that the speed of the ripple event was vastly greater
> than the speed of travel of the train. That is, the wave event was its own
> event.
>
> Does anyone know of a technical account of what is going on to account for
> these speed differences? Does the ripple speed up as it goes down the train?
> It certainly seems to.
>
> Anyway, I pose my problem in order to free my brain up, a little.
>
> cheers
>
> keith russell
>
> newcastle OZ
>
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