Andrew wrote:

Do you mean the buttons to open the doors ? If I remember correctly
these
were used during winter months, but during warmer months train
operators
were instructed to open all doors at station stops to help with
ventilation.

This is remembering from 3+ years ago when I was last in London, so
things
may have changed since then..

Andrew.

The buttons don't seem to be used at all nowadays. At a normal stop
all the doors open and the button illuminates, at terminals this
happens then the door buttons go off.

Occasionally the drivers do something at the terminals which makes only
one door per carriage stay open, although I don't know what this is but
it keeps it nice and warm

--
Chris