M A Frequency.

It'd take a wee while for this poor theologian to do the maths, but I suspect it is something to do with avoiding resonant frequencies and harmonics with the autosignalling feeds on the Hainault loop.

The 850Hz became something of a 'standard', did it not? Is it still the same frequency today?

When I was doing the research into getting my 62 TS car lights to work from the mains, I was told by the late Bob Greenaway that the engineers at Acton took an existing DC/DC machine and rewound the driven end with AC coils to produce a motor/alternator. 850Hz just happend to be the resulting frquency, so that' s what was used. The strip lights don't care about frequency or voltage, providing that the right values are used in the ballast/choke circuits. I retrofitted mine with ballast/chokes salvaged from domestic light fittings, and the original strip lights work fine at 230V 50Hz. The only catch is to make sure the ballast/chokes are appropriate for the length of tube, so they are different between the car lights and those for the destination blind, etc. Ben is correct in that the 50V DC is a rectified supply stepped down from the 110V via a transformer. On mine I replaced the original (clapped out and heavy!) lead acid batteries with four 12V "leisure" batteries in series, each with its own charger to keep it topped up. ( a leisure battery is what is used in golf buggies, mobility scooters etc, has the ability go totally flat and still recover, unlike most car batteries). I think that at least a part of the whine is the 850HZ, which would put it just sharp of 5G# for the musically minded amongst you (says the drummer!)