Specifically:
In older movies, in scenes on trains, occasionally there would be a bell, clanging very fast; apparently this was to indicate that the train was going through a grade crossing.
I don’t understand why the bell ringing was so fast, as heard from on the train. It would seem to me the clangs would be few and far between, considering the train’s speed and the short amount of time its passengers would be within earshot of the crossing bell…
You’re not asking about the Doppler shift are you? That would make the interval (slightly) faster as the train approached the crossing and (slightly) slower as it receded.
The other periodicity of the sounds would be from
- The frequency of the train passing crossings.
- The frequency of the bell ringing.
Number 1 depends on if the train is in the middle of Kansas or L.A. In KS, the train doesn’t pass many signals and, when it does, is going close to its full speed meaning that the signal is only within earshot for a few seconds. In a major town, the train would be going slower and could be within earshot of the bell for 15 seconds or so.
Number 2, the frequency of the bell, is simply because, in the old days (pre-digital) it was much easier to make an alarm that rang quickly, rather than slowly.
I think the answer to your question about why is the bell clanging so fast is that, in real life, signals clanged that fast and Hollywood just duplicated the sound.
Radiowave is correct. If you took a train, you’d hear the sound of crossing bells dopplering, and they sounded just like they do in the movies.