At least one place has both 50Hz and 60Hz running down the street. Why? I have NO freaking clue, but I'm sure there was a perfectly good reason 50 years ago that's too difficult to change now. Probably the same reason they split the frequency in the middle of the country.
I'm come across 50Hz 100/200V center-tap for the 100V 50Hz Japanese stuff, and 120/208V (delta and wye obviously fed from separate substations and separate services, I mean, why would you possibly want to feed these from a common switchboard???) in both 50Hz and 60Hz.
