What does a yellow signal usually indicate?

A yellow signal is a crucial indicator in railway signaling systems, and it typically signifies caution. When a yellow signal is displayed, it alerts train operators to prepare to stop as they approach either a red signal or another situation that requires them to reduce speed. The purpose of this signal is to ensure safety by giving train crews advance notice that they should be ready for a potential stop, thus helping to prevent incidents.

In the context of the other options, proceeding at maximum speed would be unsafe, particularly when approaching a stop signal, as it ignores the essential warning that a yellow signal represents. Stopping immediately is not the correct action at a yellow signal, since it suggests an immediate halt rather than preparing to approach the next aspect. Likewise, a clear passage ahead is represented by a green signal, not yellow. Therefore, the function of a yellow signal is firmly established as a caution signal to prepare for a stop, making it the correct answer in this scenario.