What condition must be met for Form L's related to a regular train to become void?

The condition that must be met for Form Ls related to a regular train to become void is when the train loses both right and schedule. Form Ls are used to communicate specific instructions and authority for train movements, and they ensure that an operation can proceed safely and efficiently within the established parameters.

When a train loses both its right (the authority to occupy a particular segment of track) and its schedule (the established time for arrival and departure), it indicates that the train is no longer in compliance with the operational plan. This loss of authority and timing necessitates that the Form L becomes void to prevent any misunderstandings or safety issues. The crew would need to reestablish authority and possibly update the operational parameters before proceeding.

This emphasizes the importance of communication and authority in railway operations to ensure safety and coordination among trains and crew members. The other options do not directly address the circumstances under which Form Ls would specifically become void, as they relate to either speed, route completion, or crew changes, none of which inherently affect the authority granted by the Form L in the same way as the loss of right and schedule does.