Which of the following is a requirement before a train is authorized to leave a siding?

Before a train is authorized to leave a siding, it is essential that the main track switch is lined back to normal. This procedure ensures that the train will be directed onto the correct main track, allowing for safe and efficient operations. The normal position of the switch indicates that the track is aligned for the train to proceed without encountering any misalignment or potential hazards that could result from an improper switch setting.

While the other options may involve important safety and operational protocols, they do not directly pertain to the immediate requirement for a train to safely transition from a siding to the main track. For instance, verifying that all passengers are accounted for is crucial for passenger safety but does not affect the train's ability to leave a siding. Similarly, inspecting equipment for defects and reviewing track warrants are important but are part of broader safety and operational practices, rather than a specific requirement for changing the switch position. Thus, ensuring the main track switch is aligned correctly is the critical step that must be completed before a train can leave a siding safely.