Hmm
Just remembered london underground used to use high frequency a.c. (and possibly british rail) when flourescent lights were first introduced. this was to keep size of rotary dc to ac convertors down & also to reduce the flicker effect from the lights (as the sine wave hits zero volts) in an enclosed space. 50hz was/is thought to be connected to epileptic fits. this does not affect filament bulbs. wouldn't surprise me if the same thoughts were applied as a factor in a/c