When movements are made under the provisions of Rule 6.6, which safety measure must be taken first?

When movements are made under the provisions of Rule 6.6, obtaining additional track authority is the first safety measure that must be taken. This step is crucial because it ensures that the train has the proper authorization to occupy the track in question. By securing the necessary track authority, the crew confirms they are permitted to move on that section of track, thereby preventing potential conflicts with other trains and maintaining overall safety on the rail network.

The other options, while they may be important safety measures in general operations, do not take precedence over ensuring that the train has the correct and up-to-date authority to proceed. This foundational step is integral to situational awareness and risk management within railroad operations. Without confirming track authority, the subsequent steps could lead to safety violations and dangerous situations on the track.