Could be for detraining reasons in an emergency but they
don't need to light the whole tunnel for that. Tho it seems
to be an unofficial policy on the railways to waste
as much electricity as possible. How many times have you
been on a train in bright sunlight and the interior lights are
on when theres no tunnel for miles if at all? And lets not
forget the new "efficient" 377s brought in on Southern
and elsewhere than use so much more power than the
old slam door stock that they had to build new substations!
Very enviromentally friendly. I remember when I was
younger that they used to switch the lights off fairly
frequently on the tube when the train was above ground
but they don't seem to bother now.

B2003