How is a manual block characterized?

Answer: B — segment of main track between two block limit signals

A manual block is characterized as a segment of main track between two block limit signals. This definition emphasizes that the manual block is defined by the presence of signals that indicate the limits of the block, rather than relying solely on automatic signaling systems. In a manual block, train movements are typically governed by the rules and regulations that require the conductor or another designated personnel to manage train movements between those signals, permitting an organized flow of traffic while ensuring safety. The other options highlight characteristics that do not accurately define a manual block. For instance, while automatic signals might be defined by specific lengths of track, a manual block specifically pertains to the control and management through block limit signals rather than automatic control. Additionally, the context of storage or exclusive use by passenger trains does not align with the operational functionality of a manual block, which is centered on train movement control between specified signal limits.