Don't forget the big GG1's used on the Pennsylvania railroad passenger services.
These electric loco's served for about 50 years ( 2 CO CO 2 ). 11 kV 25 Hz.

The Milwauky railway had also some interesting electric loco's with a very high tractive bipolar motor design, i read about, cant find the book at the moment.

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The lower the frequency the better for power, that is on some locos you can fit a smaller motor at 25hz than at 60hz the same HP. On a locomotive or multiple unit type train you had limited space to contend with.

Paula, I think that you got the frequency data wrong. The lower frequencies like 16 2/3 and 25 Hz were used in the earlier AC traction days when sparking at the commutators was a big problem in the motor design.
Now 50 or 60 Hz is not a problem anymore because designs have improved and even 3 phase is available via special converters built into the locomotives , which take 1Ø from the grid and via thyristors and dc bus can be converted to 3Ø for brushless asynchronous traction motors.

In the airforce 400 Hz is used, In general the higher the frequency the more compact the motors and transformers hence less weight.

Another 3 coupled units were used in Sweden
Dm3 1-D+D+D-1 used on the iron ore line. 15000 Volts 16 2/3 Herz with Jack rods on the big traction wheels.

I see if I can scan some piccies in from these loco's. I'm an electric and diesel loco fan too. [Linked Image] [Linked Image] [Linked Image]

The product of rotation, excitation and flux produces electricty.