Why would they replace generators with alternators, when the current has to be rectified back to DC for the traction motors?

If the AC / DC transformation magical technology used in the data center where I work is any example, DC doesn't have any franchise on creating waste heat. I spent yesterday doing PMs on huge air handlers, each of which keeps just one UPS cool and functional (ironically, an air handler motor overheating was the only mechanical breakdown I encountered). We're talking TONS of waste heat here, guys....and this is static equipment, nowhere near as robust as what would be necessary for railroad use.

I chalk it up (in railroading anyhow) to the desire to include complex solid state electronic controls into systems which were perfectly controllable with manual systems historically in locomotives. Perhaps the idea is to reduce human requirements for operators, or to try to squeeze out another iota or two of efficiency. Certainly there is less to maintain in terms of brushes, linkages, etc but when those things wore out or broke any idiot could see what was wrong - not so much in a big SCR or a flaw on a bit of a circuit board. Try to jam a stick in a tiny modular sealed control relay and limp home........

It's interesting to see the longevity of the GP38 and GP38-2s, long after many of their more "modern" siblings have met the scrapper's torch. I guess the locomotives remain popular with both Class I and smaller users due to the lack of turbo, and it is just the "right" amount of horsepower.