Ian Rivett wrote:
The black and yellow plate is called a "Weak Field Switch". It
allows more current to the motors to allow them to go faster. It
used to be fitted to all trains like 38 stock, R stock, Q stock etc
as well as the A stock and was a method of going faster when
needed. The idea of it being visible is that it can be seen by the
train controllers and drivers can get into trouble for using it
when not authorised. On the Met line it is used all the time.

More modern stock since the 60's I don't think have this
arrangement. I've never seen one on a 67 or 72 stock or anything
newer.

They are fitted to D stock (1978) on the District Line, as explained at
http://www.trainweb.org/districtdave...d78_stock.html

--
Richard J.
(to e-mail me, swap uk and yon in address)