Which of the following terms describes the directionality of traffic on two main tracks?

The term "current of traffic" is the correct choice as it refers specifically to the established flow or direction of train movements on two main tracks. This concept is crucial in railroad operations, as it dictates how trains are to be scheduled and routed on those tracks to ensure safety and efficiency.

Understanding the current of traffic helps dispatchers and conductors manage train movements, reducing the risk of collisions and optimizing the use of the tracks. It establishes which direction trains are permitted to travel on each track and can vary based on operational needs, such as peak travel times or maintenance activities.

The other terms, while related to rail operations, serve different purposes. A block signal indicates the occupancy of a block of track, directing trains on their positioning but not specifically outlining the directionality of traffic. A control point refers to a location where the movements of trains are managed and regulated. Interlocking pertains to a system that prevents conflicting train movements, ensuring that track switches and signals operate in a coordinated manner, but it doesn't directly define the direction of traffic flow.