In , krw wrote in part:

From what I can tell, most US systems are 60Hz since they can buy
standard equipment. Some older systems are strange for historical
reasons (generation equipment). I believe the NYC subway system is
25Hz, or some such. I'd be very surprised if any used 400Hz since
saving weight isn't as important as reducing losses.

If a transformer designed for 60 Hz is operated at 400 Hz, losses would
be less than at 60 Hz, although output voltage with a heavy load may droop
if not much work went into reducing leakage inductance. 400 Hz
transformers at 400 Hz also normally have less losses than 60 Hz ones of
same volt/amp ratings being operated at 60 Hz. And a 400 Hz transformer
would cost less to make than a 60 Hz one of the same ratings, since it
would be smaller and use fewer turns of wire.

The way I heard it, a lower frequency was preferred for motors and
that's why 60 Hz was chosen. Also, 400 Hz could be high enough for wire
inductance to be a problem with longer distance transmission lines, but I
don't know how badly or if transmission line inductance affected 60 Hz
being chosen for our main AC power frequency.

- Don Klipstein )