I assume that hoods on headlights are to reflect light downwards to the track, instead of disappearing into the air above.
I have no idea on the mounting location, but as mainline standard gauge steam seemed to get lower headlights over time, I'm going to guess it's something to do with having the light closer to the ground to illuminate the track better on large steam locomotives. However, since Mexican engines had center-mounted headlights...it was probably just preference for one or the other by the railroad.
Anyone who wants to correct me, go right ahead!