What must be included in the designation of an extra train?

Answer: B — The direction of travel

In order for an extra train designation to be clear and effective, it must include the direction of travel. This is crucial because it provides critical information that helps all personnel involved—such as conductors, dispatchers, and other train crews—understand the intended path of the train. Knowing the direction ensures that all operations can be coordinated around this movement, thereby maintaining safety and efficiency on the railroad. While other factors like the time of departure, the number of cars, and the estimated arrival time could be important for scheduling or operational logistics, they do not specifically address the immediate logistical and safety needs related to the train's designated route. Hence, the direction of travel stands out as the primary requirement for the designation of an extra train.