Under what condition may a train operate according to the next fixed signal governing movement?

The train may operate according to the next fixed signal governing movement when the signal is plainly seen. This condition ensures that the crew can clearly observe and interpret the signal's indication, which is crucial for safe operations. A clearly visible signal allows the engineer to make informed decisions regarding speed, movement, and any necessary precautions.

The requirement for visibility emphasizes the importance of communication between the signal system and the train crew. A visible signal indicates that the track ahead is either clear or that specific actions should be taken, whereas obstructed or poorly observed signals could lead to misinterpretation and potentially unsafe situations. Thus, ensuring that the signal is plainly seen is a fundamental aspect of railway operations, promoting safety and efficiency.