Why should unused sections of track be operated over at regular intervals?

The correct choice is that unused sections of track should be operated over at regular intervals to prevent rust from building up on the rails. When tracks are not in use, moisture, dirt, and other environmental factors can cause oxidation, leading to rust formation. This rust can weaken the structural integrity of the rails and lead to deteriorated conditions, which may compromise safety and operational reliability when those tracks are needed again. Regularly operating over these sections helps to keep the rails clean and lubricated, maintaining their condition and ensuring they remain safe for train operations.

The other choices, while they highlight important practices in track maintenance and safety, do not focus specifically on the rust issue. Ensuring tracks are ready for emergency use, conducting maintenance checks, and improving overall system efficiency are all crucial considerations, but the specific reason tied to preventing rust is what directly addresses the necessity for periodic use of unused track sections.