When may a train operate according to the indication of the next fixed signal?

The correct reasoning for when a train may operate according to the indication of the next fixed signal involves the visibility of that signal as well as the aspects displayed by the previous signal.

When the next signal is plainly visible, it allows the crew to assess the situation ahead and to proceed accordingly. This means that the crew can clearly see the indication of the next signal, minimizing the risk of misunderstanding what is expected of the train.

Furthermore, the indication of the previous signal also plays a crucial role. It serves as an essential cue for safe operation. If the previous signal permits movement, it provides the necessary authority to proceed, combined with the condition that the upcoming signal is also visible. Together, these conditions ensure the safe and efficient movement of trains.

Thus, both the visibility of the next signal and the aspect of the previous signal must align to ensure that operational safety protocols are adhered to, making the combination of these factors critical for the train's movement.