I enjoyed this old film. It was quite a project to build two hydroelectric plants, transmission lines, along with substations and converter stations to supply 3000 volt DC power to the railroad lines. I wonder why 3000 volt DC was chosen for the traction motor power. I suppose there are a couple of lines of reasoning: if higher voltage DC power were used, smaller diameter copper conductors could be used. Or, 3000 volts DC is still a high enough voltage to enable it to be transmitted along the railroad overhead wires for some distance between substations. The time frame for this electrification project was such that practical and reasonably sized rectifiers likely did not exist. By the time the Pennsylvania RR and the New Haven RR were electrifying their lines, they used high voltage AC in the overhead wires, and utilized on-board transformers and rectifiers within the locomotive for powering DC traction motors.
I recall seeing a GE advertising film made somewhere in the same time frame as this film. In that film, GE takes an electric locomotive and couples up to a heavy steam locomotive for a 'pull off' or 'tug of war'. Needless to say, the electric locomotive dragged the steam locomotive with its drivers turning in the opposite direction.
Electric traction is probably the best system in that the locomotive does not have to carry its own 'powerplant' or 'prime mover'. All the space and weight can be devoted to driving the wheels rather than first having to make the power to drive the wheels. Electric traction had it all over steam, better adhesion, able to take tighter curves, better ability to break loose and start a heavy train.
In other threads on this 'board, the de-electrification of the former Milwaukee Road trackage has been discussed. In my opinion, the Milwaukee Road was ahead of their time with their electrification, and in our modern era of 'going green', the idea of regenerative braking would be right in line with today's thinking. I know some corporate sharks and 'maneuvering' resulted in the de-electrification, probably seeing a short term cash infusion from the copper scrap. Now, talk of improving US rail service and 'going green' might have made the old Milwaukee Road trackage a viable thing.
The box cab locomotives are really classic, and obviously did their work quite well. I know that after WWII, the Milwaukee Road acquired another batch of electric locomotives called 'Uncle Joes'. These had bodies that approximated a GG-1. The story was these locomotives had been built for shipment to the Soviet under maybe the 'lend lease'. Either the war ended, or some other factors intervened, and the shipment to the Soviet was cancelled. Because the locomotives had been originally ordered for the Soviet Union, they were dubbed "Uncle Joes" after Joseph Stalin. The obvious thing was that the locomotives had to be re-gauged to standard gauge (Soviet railroads use a wider gauge). Whether these locomotives were setup for 3000 volt DC overhead power, or whether there had to be some modifications made would be interesting to learn about. I know the Milwaukee Road used a number of the 'Uncle Joes' probably to the end of their electrified operations.