When is it permissible to display a green flag?

Displaying a green flag serves a specific purpose within rail operations. It is an indication to the crew that they are no longer subject to yellow-red flag restrictions, effectively allowing them to proceed as indicated. The green flag conveys that previous cautionary signals are no longer in effect, thus reverting the train's status to a regular operating condition.

The other choices describe different signaling purposes. For example, signaling a change in track direction typically involves other indicators or flags, and is not designated by a green flag. Indicating a temporary traffic stop usually requires a different signal entirely to effectively communicate that all movement should cease. Similarly, alerting the crew of upcoming restrictions is communicated through distinct signals that convey the nature of the restrictions that need to be observed, rather than utilizing a green flag. Thus, only the first option aligns correctly with the protocol for displaying a green flag in maintenance of way operations.