What happens to the signal governing entrance to a track for an immediate movement on rusty rails?

The correct choice highlights a key protocol regarding signal indications in relation to track conditions, particularly on rusty rails. When a signal is governing entrance to a track that has not been maintained, such as with rusty rails, there are inherent risks involved due to the potential for reduced traction and stability. To mitigate these risks, the signal must be displayed specifically for an immediate movement.

This is crucial for ensuring the safety and efficiency of train operations, as immediate movements allow for careful monitoring and adjustments in response to the condition of the track. By limiting the indication to only immediate movements, it ensures that trains do not proceed onto the track for a longer or extended journey, which could lead to hazardous situations.

In contrast, the other choices either suggest a level of readiness for longer movements or an absence of signal requirements, which would not prioritize the necessary caution needed when dealing with potentially unsafe track conditions. Therefore, emphasizing immediate movements in this context helps uphold safety standards in train operations.