What signal indicates that a train must be stopped by a person on the roadbed?

The signal that indicates a train must be stopped by a person on the roadbed is when a hand, flag, or light is waved across the track. This action serves as a visual signal to train operators that there is a need to halt the train's movement. The waving motion is a clear and universally recognized signal that can be seen from a distance, thereby ensuring that the message is communicated effectively.

Using a hand or flag provides a method of signaling that is less prone to misinterpretation compared to other means like vocal shouts or horn signals, especially in noisy environments or situations where visibility may be impaired. Additionally, the waving across the track can be readily distinguished from other forms of movement, reinforcing its specificity as a stop command.

While other options might represent forms of signaling, they do not offer the same clarity or reliability as a clear hand, flag, or light signal during train operations.