120 V 230 Volts mains, which is best?

[colly0410]

When I was on the London underground & southern electrics many years ago the lights would dim & brighten when accelerating as the starting resistances were switched in & out. When on the Nottingham tram last year the lights were a constant brightness while accelerating & regenerative braking, must have a very good voltage regulator control gubbins...

I'm wondering why it doesn't use AC instead of DC, must be a good reason that I don't know about...