Which definition best describes "main track" in the context of locomotive operations?

The definition that best describes "main track" in the context of locomotive operations is indeed the one that emphasizes a track authorized for movements controlled by block signals or interlocking rules. Main track is crucial in the railway system as it is typically where the primary movement of trains occurs, allowing for more controlled and efficient operations.

The use of block signals or interlocking rules on a main track ensures that trains can operate safely and efficiently, with signals indicating when it is safe to proceed. This system helps prevent collisions and manage train flow along busy routes. The characteristics of the main track differentiate it from other types of tracks, such as sidings or storage tracks, which do not manage train movements in the same way.

Conversely, a track where speeds are unrestricted, while it might suggest a certain level of freedom, does not encapsulate the regulations and safety measures inherent to main tracks. Tracks designated exclusively for storage do not facilitate the active movement of trains, which is a core function of a main track. Lastly, a segment of track with no signaling control does not meet the safety requirements for main track operations, as it lacks the necessary signals to guide train movements safely.