What must not be considered closed except as provided for by specific authority?

The correct answer signifies that timetables or train orders are critical documents within railroad operations that establish rules and procedures for train movement. These documents dictate how trains should operate, including schedules, train priorities, and any specific instructions that may affect train service.

In railroad operations, these documents must always be regarded as open to ensure safety and compliance with operational standards. Only authorized modifications or cancellations can officially close them. The implication is that without explicit authority for closing or modifying these documents, the existing rules remain in effect. This ensures that all personnel adhere to established protocols, thereby minimizing the risk of accidents or miscommunication.

While other aspects, such as signal indications or dispatcher authorizations, play a role in train operations and safety, timetables and train orders are fundamental to the overall structure of an operation and are specifically treated with this level of importance regarding closure. This ensures clarity for all train operations and personnel involved, ensuring adherence to the operational framework established by the railroad.