What must be in effect for movements against the current of traffic?

For movements against the current of traffic, having an authorized track warrant or bulletin is essential as it provides the necessary permission and specific instructions to ensure the safety of the train and other rail operations. These documents serve as official communications that outline conditions and restrictions pertinent to the specific track and time frame, allowing for safe navigation against the established flow of rail traffic.

The need for a track warrant or bulletin arises from the requirement for strict adherence to protocols designed to minimize risks and maintain operational safety. By having this authorization, conductors and crews can ensure that all safety measures and directives are acknowledged and followed, effectively reducing the potential for accidents or conflicts with other trains.

The other options do not substitute the need for a formal authority like a track warrant or bulletin. While signals may indicate conditions, they do not replace the requirement for an official order when moving against traffic. Similarly, reports from the crew or verbal agreements do not provide the same level of clarity and authority needed to ensure safety in such operations.