I have one booster unit with a "backup light" -- a powered Atlas Erie-Built B-unit (which also has sound). It can be hostled like the real 90A, with the bell ringing and the engine revving.
In real life, when railroad ordered cab and booster units, a rear headlight on blind ends was optional, and, if used, was, in most cases, applied by the railroad. On the Santa Fe, our F7B's did not have headlights. However, the railroad did have some portable headlights which could be hung on the booster unit (or on the blind end of a cab unit) and were then plugged into the m-u receptacle. The power for rear headlights comes through the m-u cables from the controlling unit of the consist, not from the rear unit. We kept such a headlight in the phone booth at the main track connection of a spur at Snyder, Texas, for crews to use if they had to go up the spur with a bobtail consist. They had to back the engine one way.
I was never on a foreign line booster unit with a permanently applied headlight, but there would have been a headlight switch at the hostler control stand inside the engine room by the center (hinged) porthole, so it could have been manually controlled when hostling the unit, and would have, like any other permanent headlight, have responded to the rear headlight switch on the controlling unit of an m-ued consist.
Santa Fe did add a rear headlight to some FTA units which were modified for single-unit local or branch line service, but the headlight was a Golden Glow barrel headlight reclaimed from retired steam engines and mounted at the rear on the roof.
