Somewhat off-topic, but I want to thank a member who suggested that this topic should be iirc in General matters. However, it's now into multiple pages, and I'm too new to this Forum to try to move the whole thread. I surely wouldn't mind a tall if some experienced and responsible person did, though. I have yet to read most of the posts.
Someone asked how DC has been switched; that has probably been answered, but I'll say what I know about it. A DC arc has a magnetic field, and one stratagem is to have a blowout magnet that forces the arc to lengthen. Long before Alnico, permanent magnets were weak, and demagnetized easily. An electromagnet, the "blowout coil", supplied the field. Pole pieces were on both sides of the arc, as I recall.
As well, there were arc suppression chambers, with multiple insulating baffles to split up the arc. This is as I remember, but only from illustrations.
The Boston T light-rail Green Line depot at Riverside, its western terminal, had a siding where 3400 series "hangar queens" were stored. Covers had sometimes been removed, and it was quite interesting sometimes to see the goodies, especially electricals. I distinctly recall that what must have been control-circuit relays had huge insulating chambers for their contacts.
Off-topic, but also of interest was an air-core power inductor which occupied maybe an 8-inch/20 cm cube, maybe even 12-inch/30 cm. It might have been wound with litz wire, have forgotten.
DC for the traction motors must have been modulated by pulse modulation, but quite likely pulse frequency, audio range, instead of width. I recall riding in 3400-series cars, and you could easily hear the "pulser".