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Trains in Norway (and, presumably, also elsewhere) run on 16 2/3Hz AC.

This is a historical artifact - back in the day, electrical motors didn't fare too well if fed with 50hz, so a lower frequency was desirable.

In airplanes, on the other hand, AC supply is typically 400Hz - as higher frequency equates a smaller, lighter transformer for a given throughput.

16 2/3 Hz or 16.7 Hz as the newer standardization says is used by all countries that used AC early. 50 Hz wasn't feasible for railways before the 1960s or so.

16 2/3 Hz is at least in use in Germany, Switzerland, Austria, Sweden, and Norway. In Germany development started already in the 1920s and regular usage in the 1930s.

France, Finland, and Denmark for example were later with AC and they chose 50 Hz.

Re: 400hz: and smaller magnets for the generators/motors.

Just like how a higher frequency antennas can be smaller for the same amount of gain.

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