Conventional operation or under DCS?
I do not have the Bigboy, only the challenger in MTH Premier. MTH 20-3687-1 O Union Pacific Hi-Rail 4-6-6-4 Challenger w/Proto-Sound 3.0 #3985
The Cab light is just above the gauges on the Engineer's side and being highly directional in nature, is not easily visible in full room light. In fact, I had my lights off in the room to validate mine.
Mine came on from startup in DCS, off when moving, on again, sitting neutral.