Cab view from 1938 tube stock on the Bakerloo

I can say this now that the individual has retired.

A Bakerloo T’Op I befriended during my work experience days used to drive with the headlights off (he called it night watch mode) save for station approaches all day on a Sunday.

I think like yourself jimbo , he was a guard who then progressed up in the last months of the 38 stock in regular service.

I was in the cab on his last few Sunday turns and for me it was all the more surreal, you really appreciate just how bright those signal lenses are and how far they carry.

To warn people on or about the line of the approach of the train

Trains used to have marker lights (usually oil) they would warn you of the approach of a train, and depending on the Railway and the date would either tell you what route it was travelling (LB&SCR etc.) or tell you the type of train i.e. Light engine, express etc. These original lights did not illuminate the track, signs etc. But this was not a big problem when you had a large kettle to keep warm, that largely blocked the view of the way ahead! Also with most trains by the time you see something you have already passed the minimum braking distance.

Modern trains have for sometime had headlamps, they are very useful if you want to see what you are about to hit. They are of course mainly used for seeing the modern reflective signs, the old oil or battery lamp ones had a habit of going out. Also if you believe the tutors on track safety courses they also allow the driver to see you and warn you of a trains approach when you are working track side.

I pointed out to staff on a preserved railway a few years ago that the oil lamps on the buffer beam don't give enough light to reflect off those stripes.  Also the stripes on older Hi-vis become less reflective as they get older, I am unsure if it is U.V. light or washing powders that degrade / wash out the reflective chemicals.

I also remind staff that if they are relying on a driver seeing them and giving a warning they have made a potentially fatal mistake.

I don't believe that there is any purpose served in illuminating the road ahead in tube tunnels. It brings the tunnel segments into view, causing fatigue, and distracts from the visibility of signals. I suppose it could allow a driver better to see a defective, unlit signal, although the advantage of that, (Ooh! I wonder if it's red or green?) seems minimal.
It would be interesting to do some research on the subject.

Headlights were first on Victoria line trains, where there were almost no light signals, so automatic train operator pressed start buttons and train rushed off through the tube tunnel to the next station if all was clear. It was feared such movement through near darkness would risk disorientating train staff. Later train deliveries had headlights ready for eventual conversion to automatic, although this never happened until the Central line.