They actually used that in the US as well for railways. I remember a post years ago on the Classic Computer Mailing List from someone who said that their father had worked on the railways and pointed out that the "high bay" station lighting all ran off 3-phase 16.7Hz power. Apparently it looked okay at ground level but was quite disconcerting when you looked up and saw the lights flickering in patterns of three.

Early AC electrification systems in the US were typically 25Hz, not 16.7Hz. Parts of the northeast corridor still use 25Hz electrification.

I'm trying to find the post, but it was definitely 16.7Hz. I don't know that it was particularly "early" as such, probably the 1950s or so.

It used to be exactly one third, i.e 16 2/3 so you can have generators on the same axle. However, being exactly one third caused unwanted resonance effects. So with the advent of power electronics it has been slightly shifted to 16.7 Hz. Within tolerance for the motors, but no unwanted resonance anymore.

My high school physics is not sufficient to really understand it.