Under what conditions must extra trains in yard limits operate?

Extra trains operating in yard limits have specific requirements to ensure safety and operational efficiency. The correct choice states that these trains must be prepared to stop within one-half the range of vision. This requirement emphasizes the importance of caution in potentially congested areas where other rail operations, maintenance activities, or unexpected obstructions could occur. By operating under this principle, train crews can effectively manage any unforeseen circumstances and ensure they can respond appropriately to maintain safety.

The other options do not encompass the fundamental requirements of operating extra trains in yard limits. For example, restricting operations to only nighttime or imposing a blanket maximum speed limit without consideration of visibility would not adequately ensure safety. Similarly, relying solely on signal indications might not account for the dynamic environment of a yard, where additional factors come into play, underscoring the necessity of a heightened state of readiness to stop as required.