What must occur for a train following another train on rusty rails?

For a train following another train on rusty rails, the requirement is to operate at restricted speed, which is indicated by a Form D line 13. This is essential because rusty rails can lead to a decrease in traction and braking effectiveness, posing a safety risk.

Restricted speed is defined as a speed at which a train can stop within half the distance of visibility or within a shorter distance specified by the dispatcher, allowing the crew to respond promptly to any hazards that may arise. Given the potential for track conditions that are less than optimal due to rust, this requirement helps ensure that the following train maintains a safe operating speed and can react to the actions of the leading train or any other obstructions on the track.

The other options suggest operating at normal or limited speed, which would not account for the increased risk associated with rusty rails. Without the specific requirement for restricted speed, the safety of the train's operation could be compromised.