Not sure this is the answer you seek, but the original wiring diagram posted on LionelSupport shows, on page 1, a "circuit board for cab illumination" towards the top right in the attached PDF copy of that page. Wiring goes back to the Motherboard as you say; I assume that the control for on/off is there.
I also assume that this was a unique motherboard specific to this model - rather like the original TMCC Veranda motherboard that had the sound and smoke controls on it. As both the Acela and the Veranda are listed in the latest catalog as Legacy versions, presumably Lionel has had to re-engineer their operating features - but that does not mean that anyone still there knows how the original ones functioned. But I agree that you'd think that would be part of the knowledge bank.
