When can a train operate according to the next fixed signal's indication?

A train can operate according to the next fixed signal's indication when that signal is displayed and visible. This means that the crew must be able to see the next signal clearly to ensure they can interpret its indication correctly. Visibility of the signal is essential because it ensures that the train is operating safely and adhering to the displayed indication, whether it is to proceed, slow down, or stop.

Understanding this rule helps maintain safe and efficient train operations since crews rely on signals to make quick decisions while on the move. If a signal cannot be seen, it is not safe for the train to operate based on its indication, as this could lead to potential safety hazards.

The other choices suggest scenarios that do not directly relate to reliable signal indication. For instance, operating based on a previous signal, time of day, or dispatcher permission can introduce uncertainty and does not replace the requirement of having a visible signal to guide the train's movement safely.