What does a flashing red light over the letter 'O' signify?

A flashing red light over the letter 'O' signifies that the train engineer must stop and call for orders. This indication is critical for ensuring safety and communication regarding the train's movement. The flashing red light serves as a warning that the train is not authorized to proceed without first receiving explicit instructions. This protocol is in place to prevent potential accidents and ensure that train operations are conducted safely and efficiently.

By requiring the engineer to stop and communicate, it allows for verification of the train's position and conditions ahead, enabling control personnel to assess the situation and provide the necessary directions. This practice is essential in railway operations where multiple trains may be in proximity, and clear communication can help avoid collisions or unsafe conditions on the tracks.