> Due to the fact that there are only pulses with different frequency
> (from lets say 0 Hz to 30 Hz) over a RF link it's not clear for me
> how to code it...

You could include a small PIC, eg 10F or 12F. What, for example,
happens if the train stops with the pulse high ? With a micro you could
count and send ASCII once per second or something like that

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