In 251 territory, what must a train obtain to back up beyond the current block?

In 251 territory, a train must obtain a Form D line 2 authority to back up beyond the current block. This authority is a formal document that allows the train to proceed or operate in a specific manner, in this case, to reverse beyond the limits defined by the current block. The Form D line 2 authority provides the necessary permission for trains to operate on sections of track, ensuring that operations are safe and authorized, especially when reversing movements that can potentially conflict with other train movements in the area.

This type of authority is essential because it establishes a protocol for communication and safety in train operations, particularly in complex track environments like those found in 251 territory. Without obtaining this specific authority, a train may not be able to safely back up, which is crucial for maintaining the integrity of the rail network and preventing accidents.