Switzerland trains still use 16,7 Hz (16 2/3Hz)

Yes indeed, but the German-Swiss-Austrian 16.7 Hz system is many orders of magnitude bigger than the Southend Electrification! The path dependency is much more understandable in that case.

It's also 16.7 Hz precisely (save for the fluctuations) and not 16 2/3 Hz, for some electrical reasons regarding transorming 50 Hz to 16.7 Hz mechanically, which I don't completely understand

I understand that historically it was 16 2/3 Hz and power transfer from to the 50 Hz grid was done via 1:3 mechanical coupling with a motor and generator. That was a ratio easily achievable with gears.

Nowadays with switched power supplies, this is not a problem any more. Keeping track of 16.7 Hz seems a little easier. Imagine building a numeric display for a power plant operator to see how far off you are.

> Imagine building a numeric display for a power plant operator to see how far off you are.

You could build a display with 3 as the denominator, and a decimal numerator:

   | |_   2.1
   | | \  ---
   | \_/   3

Ontario, Canada, at niagara falls generated 25hz for industry (stelco and others) until the early 2000s https://www.lifebynumbers.ca/history/the-rise-and-fall-of-25...

Large parts of the NE corridor train line(DC to NY and New Haven to Boston) still operates at 25hz and has its own power system and generators because of it.