I think loco exterior lighting does nothing. Almost every TSW2 & 3 routes "bright" setting barely scratches the realistic point of lighting ahead. You can't see more then a few meters in front of you before its just dark. I wish that locomotive lighting was brought up to a more realistic level. So we can actually drive and see in the dark without using the hud to see signs and such...
Remember that in a lot of cases (not all) the headlights are to let people know where the trains are, not necessarily to light the road ahead.