What defines the tracks between the extreme opposing signals of an interlocking?

The term that defines the tracks between the extreme opposing signals of an interlocking is known as "interlocking limits." These limits refer specifically to the section of track over which the signaling system of the interlocking governs train movements, ensuring that conflicting routes are not set simultaneously.

Interlocking limits are significant because they establish a safe operational area where the signals and switches are controlled to prevent accidents and ensure the smooth flow of train traffic. This is particularly crucial in areas where multiple tracks and routes converge, allowing for precise control over train movements.

The other concepts mentioned, such as railroad boundaries, track segments, and regional zones, do not specifically refer to the defined area managed by an interlocking system. Railroad boundaries typically pertain to the jurisdiction or ownership of a segment of track, track segments denote portions of the track network, and regional zones are more related to geographical areas of operation rather than the operational control found within interlocking systems. Thus, interlocking limits is the correct terminology that encapsulates the operational safety and signaling control in that defined section of track.