Under what condition may Speed Restriction using portions of a mile be used?

The application of speed restriction using portions of a mile is appropriate when it is aligned with a physical characteristic location. This means that the restriction is tied to specific features or conditions of the track—such as curves, grades, or other physical attributes that necessitate a lower speed for safe operations. By associating the speed restriction with a tangible aspect of the railroad infrastructure, it ensures that the restrictions are both relevant and justifiable based on the actual conditions present.

The other options do not accurately describe the conditions under which speed restrictions can be implemented. For instance, applying a speed restriction to all trains would not take into account varying operational needs or track conditions that could necessitate different speeds for different trains. Specifying a restriction only during daylight hours overlooks the fact that track conditions requiring reductions can occur at any time. Likewise, the presence of multiple speed signs doesn't necessarily provide the context needed for the application of portions of a mile; it is the physical characteristics that truly dictate the necessity for such specific restrictions.