When should a train operator expect to use caution due to the presence of workers on the track?

A train operator should expect to use caution due to the presence of workers on the track when a yellow signal is displayed. A yellow signal serves as a warning indication, prompting the operator to prepare for potential hazards ahead, including the possible presence of workers on or near the tracks. This signal indicates that the train may need to slow down and be alert for any unexpected situations that could arise in the area.

The other options typically convey different meanings: a red signal usually signifies a complete stop or danger ahead, a green signal indicates that it is safe to proceed, and a clear track suggests there are no immediate obstructions. Understanding the implications of these signals is crucial for ensuring the safety of both train operations and crew members working on or near the rails.