What authorizes a train to run against the current of traffic?

The correct answer is Form L, as it specifically provides the necessary authorization for a train to operate against the current of traffic. Form L is a formal document used in rail operations that details instructions and permissions for train movements under certain conditions. When a train is required to travel in the opposite direction of normal traffic flow, it's crucial to have clear and documented permission to ensure safety and coordination, especially in busy systems where the potential for confusion or conflict with other train movements exists.

This form contains critical information including the location, reason for the move, and any additional instructions that must be followed. It is designed to maintain safety and operational efficiency, allowing crews to execute unique movements while minimizing risk.

The other options relate to aspects of train movement but do not provide the necessary authorization specifically for running against the current of traffic. General authority refers to permissions that may apply broadly within the operational guidelines, verbal confirmation from the block operator may be useful for immediate situations but does not serve as formal documentation, and timetable indications typically guide normal operations and schedules rather than permitting against the grain movements. Thus, having Form L is essential for proper authorization in these scenarios.