That sounds similar to a defect detector, that is widely used in the 
USA. It counts axles, scans for hot bearings, and other defects. How 
close to the rail will this be located? Can you mechanically fasten it 
to the rail? If so an accelerometer.

Youda He wrote:
> I need to find some audio frequency sensor for an application.
> The sensor will be placed on railroad track to "listen" the sound from
> train, in order to warn people the train is approaching. I also want to
> check if the sensor can tell whether the track has problems, such as
> fracture, assuming I also had a signal generator at certain distance on the
> track, produce a impulse or other signal (like hitting the track using a
> hammer), by receiving the signal, I can do power spectrum analysis to find
> out if there are fractures on the track.
>
> Any idea on what type of sensor are available there?
>
> -- Youda
>   
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