While doing a bit of idle research into the upcoming Standard Stock
(aka 'S' Stock) being designed for the Underground's Sub Surface Lines,
I came across an interesting note which stated that regenerative
braking is to be provided and used by the S stock on the SSL lines, and
_only_ on the SSL lines - the S stock running District Line services to
Richmond and Wimbledon will not regenerate when they are on NR metals.

I'm guessing that the trains will achieve this by using some kind of
mechanism which can detect the voltage level of the negative pole of
the traction circuit and subsequently cut in the regen mechanism when
the negative pole is within the appropriate voltage range. Does anyone
know how this could be accomplished?