Which type of flag indicates when a train may be required to stop?

The yellow-red flag serves an important role in train operations as it signals that a train may be required to stop. The presence of this flag indicates caution and alerts train crews to potential hazards or issues ahead that necessitate a stop. It is used in various situations where safety protocols dictate that a train needs to reduce speed or halt completely, allowing for necessary inspections, maintenance, or other operational considerations to occur safely.

In contrast, flags like the blue flag are typically used to indicate that maintenance work is taking place on or near the tracks, allowing for safe working conditions, while the green flag usually signifies that the track is clear or that it is safe to proceed. The black flag does not commonly serve a specific signal related to stopping, which distinguishes the yellow-red flag as the correct choice in this scenario.