Which term describes a block of cars that has been off air for an extended period?

The term that describes a block of cars that has been off air for an extended period is "Off air block." This terminology is used specifically in the context of train operations to indicate that the air brake system is not operational due to the lack of pressurized air in the brake lines. When a block of cars is off air, it means that the air brakes cannot function, which is an important safety consideration.

Understanding this term is essential for anyone working with air brake systems, as it highlights the need for proper inspection and management of air pressure in train operations. Recognizing an off air block allows personnel to take necessary precautions, such as re-establishing air pressure before attempting to move the train or perform maintenance on the cars. The other terms, while they may describe different aspects of train operations or braking, do not specifically address the state of the air brakes in this context.