Final answer:
The handbrake indicator light on the train operator's auxiliary control panel illuminates when the handbrake is engaged.
Step-by-step explanation:
The handbrake indicator light on the train operator's auxiliary control panel illuminates when the handbrake is engaged. The handbrake is a mechanism used to keep the train stationary or prevent it from rolling. When the handbrake is activated, it triggers the indicator light to inform the operator that the handbrake is engaged.