Final answer:
The most likely cause of a dark train operator's indication is a failure in the electrical system or a malfunctioning light source.
Step-by-step explanation:
The MOST likely cause of a dark train operator's indication is a failure in the train's electrical system or a malfunctioning light source. This could be caused by a blown fuse, a faulty bulb, or a wiring issue. To fix the dark indication, the electrical system or the light source needs to be repaired or replaced.