Which territory requires a Track Authority form to operate against traffic?

In railway operations, Rule 251 territory is specific in its requirements regarding the movement of trains. In this territory, trains are authorized to operate in either direction on a single track that is signaled for one direction of movement at a time. When trains need to operate against the established traffic flow, which is typically governed by signals, a Track Authority form becomes necessary to ensure safe and coordinated operations.

The purpose of the Track Authority form is to formally grant permission to a train to move in the opposite direction of traffic, essentially overriding the normal signaling system that restricts such movements. This helps to prevent potential collisions and ensures that all relevant personnel are aware of the operation against traffic, contributing to overall safety on the railway.

In other types of territories like Rule 261, where trains can operate in both directions without the need for a Track Authority when following signal indications, or in Non-Signaled territory, where different operational rules may apply, the necessity for this specific form does not typically arise. Main track territory may have its own operational rules distinct from what is required in Rule 251 territory. Thus, the requirement for a Track Authority form specifically pertains to situations encountered in Rule 251 territory, making it the correct answer.