What must the Dispatcher issue to the helping train when assisting a disabled train in 251 territory?

The correct requirement for the Dispatcher when assisting a disabled train in 251 territory is to issue Form D line 2. This form is specifically designed for the situation where a train is being assisted by another train. In 251 territory, where train movements are regulated by signal indications and there are certain restrictions on track usage, Form D line 2 provides crucial permissions for movement toward the disabled train.

The issuance of this form allows the helping train to operate on the track and helps ensure the safety and coordination of train movements in a territory where space and signals dictate the operational parameters. It lays out the necessary conditions under which the assisting train can proceed, therefore maintaining the safety and integrity of train operations while addressing the incident of a disabled train. This form is a vital communication tool between dispatchers and train crews, ensuring that everyone involved is aware of the operating parameters and safety considerations in place.