What must be done to determine that all couplings have been made?

To determine that all couplings have been made, a crucial step is to change the engine direction to remove any slack in the train. This process helps ensure that the couplings are properly engaged and makes it easier to identify any issues that may exist, such as uncoupled cars or loose connections. When the engine moves in a different direction, any slack that may be present in the train string is eliminated, allowing engineers and crew members to confirm that the connections between the cars are secure and that the couplings have been made correctly.

While signaling among employees and conducting visual inspections are important safety protocols during switching operations, they don't specifically test the integrity of couplings in the same direct manner as changing the engine direction. Audible signals also play a role in communication, but they don't address the physical connection of the couplings. Therefore, adjusting the engine direction to remove slack is the most reliable method for confirming that all couplings have been made securely.