Thanks for the quick answers...

I cannot see a camera lens in there - I had thought of that purpose.

As for the detrainment, possible but seems unlikely. It is two high
(top most part of the window) and I would have expected a halogen it
is was for emergency.
There is some stock, district line I think, which has a small 50w(?)
halogen in a clear bubble pointing down. This would illuminate the
step out the from of the cab.
The one on the pic would do a good job of lighting the roof


Cheers,
Crispin