Eventually I'll get to actually rolling a POC/tech demonstrator that just has less modules at perhaps less current density, for showing that even several kV DC

Eventually I'll get to actually rolling a POC/tech demonstrator that just has less modules at perhaps less current density, for showing that even several kV DC can be efficiently transformed not just on paper to few or sub kV DC. At enough voltage grounding is no longer optional anyways, so might as well do essentially an auto transformer plus extra protection to protect humans against electric shock (RCD doesn't work directly, but the functionality can still be offered, it just has to sense quite differently). Why DC? An overhead line only limited by peak voltage (arc) and thermals can carry twice the power when running DC instead of AC, assuming both measured relative to ground. Also, you can run you transistors completely steady-state at all frequency components between their own switching fundamental and your load transients. No more over provisioning just to make up for legacy 50/60 Hz AC. Also, to a degree, you can just plug raw batteries in with that be DC grid, at most having a little bit of DC regulation to force the voltage a bit higher/lower than the batteries. Like, a power supply basically rated to a couple percent of the battery input/output max power: only need to move the small extra voltage, though ofc at the full current. Lastly, DC converters are just way smaller and lighter, so you could avoid the heavy bulky transformers in trains and alleviate power limiting from them. Relevant for fast double-decker trains because you'd prefer to have human space where you currently park the transformer. I have to say though, novel development of technology by pulling recent innovations in the fundamental/material science fields underlying the target, is very not an easy thing to do.