Great 1915 film by GE promoting electric locomotives, using the MILW through the Rockies for its theme. The intro is lengthy, and you wonder what's the point. Then it gets into the meat and becomes interesting.
Neat! That's something about the "regenerative" feature. I wouldn't have thought it that complicated then.
It's far easier with DC than with AC. But regulating voltage with 1915 technology was all mechanical relays, so not precise as it is with today's computers. Voltage regulators in 1940-50 automobiles required up to three relays, and were notorious for failing.