In message , at 22:34:04 on Mon, 27 Jan
2020, Clive D.W. Feather remarked:
So the impression I got was that the wiring could handle 750V (that's
just insulation) but the motors couldn't and so you had to keep some
resistance in the circuit.

Surely the breakers went because the current was too high, not the voltage?
Of course, the current could become excessive because of the higher
voltage.

Indeed: Ohm's Law applies.

(Though what I learned at school is Ohm's actual law was that resistance
is constant for most materials and therefore V is proportional to I.)

Ironically, today's trains (diesel as well as electric) rely heavily on
materials where the resistance is *not* constant.
--
Roland Perry