Thanks for the detailed explanation of why the railroads stuck to 25 Hz for AC motors instead of just using 60 Hz. However, no good deed shall go unpunished so please accept a couple of questions.
1. Why is a lower electric frequency (i.e. 25 Hz) more suited to powerful motors than a higher one?
2. You say that 25 Hz was used up to the 50's. Were new technologies developed at that time to allow the use of 60 Hz?
Answer to #1:
A couple of reasons. The inductive reactance of the field and armature windings becomes a significant part of the overall voltage drop across the motor - in an ideal series motor, the voltage drop is solely due to "back EMF". This is why my uncle's razor ran faster on DC than AC. The more serious reason has to do with commutation. On a DC series motor, the interpole compensates for armature reaction and thus allows the motor to be operated at a wide range of currents without having to adjust the brush position. An an AC series motor, the interpole does a poorer job of compensating for the combined effects of armature reaction and inductive reactance - where the inductive reactance increases with frequency.
Commutation problems increase with increasing terminal voltage, the motors on the GG1 were rated at 235 to 250 volts, where similar sized DC motors could be rated up to 1500 volts. The PCC's used two motors in series to allow use of 300 volt motors in the interest of better commutation. The downside of lower voltages is that the current increases - on the GG1, each motor was drawing over 2,000A when operated at its short time rating.
AC series motors need to be built with a laminated frame to reduce problems with eddy currents.
Answer to #2:
The mercury ignitron was developed to the point where it was practical to use in on-board RR service - mercury arc rectifiers where used on the South Shore Line in 1926 when it was converted to run on 1500VDC for compatibility with the Illinois Central commuter electrification - the rectifiers were in substations. Ignitrons work just as well at 60 Hz as they do at 25Hz. The use of ignitrons on locomotives and MU cars allowed those vehicles to be equipped with DC traction motors, which were smaller, lighter, more efficient and cheaper than AC series motors.
Silicon rectifiers became available about halfway through the E-44 production run, which made operation off of 60 Hz even more practical.
For a given power rating (actually volt ampere rating), the weight of a transformer is inversely proportional to the lowest design operating frequency. A 25 Hz transformer will need roughly 2.4 as much iron as a 60 Hz transformer (60/25 = 2.4). 400 Hz transformers will require 60 to 7 times less iron than a 60 Hz transformer, which is why aircraft power systems run on 400 Hz.
The size of an AC induction or synchronous motor is roughly proportional to its torque. Since the shaft power of a motor is the product of motor speed and torque, a high speed motor of a given size will almost invariably produce more power than a low speed motor. That's why a 400 Hz motor can produce more power for a given weight than a 60 Hz motor.