When may a train comply with the next signal's indication?

The correct choice emphasizes the importance of visibility in train operations. A train may comply with the next signal's indication when the signal's aspect can be clearly seen. This means that the crew must have a clear visual confirmation of the next signal, which is crucial for safe navigation and decision-making on the tracks. If the signal is visible and indicates it is safe to proceed, then the train can appropriately follow that indication.

Visibility takes precedence because it allows the crew to assess not only the signal's aspect but also surrounding conditions that may affect train movement. While other circumstances could also contribute to a decision to proceed, such as clear tracks or previous signal indications, the fundamental requirement is that the upcoming signal must be visible and readable to the crew before any action is taken.